It is said that in Germany asking for tap water in a restaurant is rude and looks stingy - examples in here and here. But I don't like to purchase water there as buying it in a restaurant would likely be too expensive since they can exploit it in their favor (at least that is true in Asia).
So I wonder if it is rude to buy it somewhere like a convenience store or supermarket in advance and bring it to a restaurant. Or is there anything others you can drink in cheaper price while eating?
they can exploit it in their favor
Everything in restaurants¹ is more expensive than the pure cost to acquire or produce it. This is how the waiting staff and location is paid. The only difference in Germany is that there is no culture of offering free tap water everywhere.
Thus asking for tap water in a German restaurant (and expecting to pay less than for bottled water) is like insisting on only paying the wholesale price for any other beverage. On the other hand, if you are willing to pay as much for tap water as you would pay for bottled water, you are at worst considered eccentric.
So I wonder if it is rude to buy it somewhere like a convenience store or supermarket in advance and bring it to a restaurant.
Most restaurants¹ have a house rule that explicitly forbids you to consume food or beverages that you brought.² In cheaper establishments, you may find big signs telling you about this; elsewhere it is small print.
¹ And any other place that sells you food and provides a place for you to consume it.
² The main exceptions to this are many beer gardens, most canteens, and restaurants that won’t serve alcohol for religious reasons but allow you to bring your own for a fee.
Or is there anything others you can drink in cheaper price while eating?
If you are with a group, it’s fully acceptable to order a huge bottle of water with a few glasses and share the price.
Except for this, the answer is that you are expected not to go to restaurants if you want to save money. Of course, I am aware that this may not be possible for you, e.g., if you are with a group, but this is the general attitude towards this: Going to a restaurant is considered a (small) luxury; if you want to eat and drink cheap, you are expected to go to a supermarket, fast-food vendor, or similar. Note that while for fast-food restaurants and similar, the aforementioned rule usually still applies, you can almost always take the food and consume it elsewhere with your own water.³
Note that the waiting staff in Germany doesn’t rely on tips for their living. Therefore a less rude way to save money is to omit the tip, in particular if you provide an excuse for it (or it is obvious that you were dragged along with a larger group, who tips well).
³ The best way to do this in turn is to have one or two bottles that you re-fill with tap water at every opportunity, such as in your residence.
While much has been said about the price and legal aspects in other posts, I am trying to give a bit of a cultural perspective in this answer (at least as far as I perceive it, as a native German who regularly likes to go to not-too-expensive (meals between 10 and 15€) restaurants with varying groups of people).
In German restaurant culture, drinks are considered about as important as food.
Typically, a short while after guests have sat down at the table and received the menus, a waiter will ask them what they would like to drink. Guests are then given some more time to study the food part of the menu before the drinks are served and the waiter will ask for food orders.
When the waiter arrives to accept drink orders, it is kind of expected that everyone will order a beverage to consume along with their food. It is certainly not unheard of that someone will only order some food, but it is slightly unusual (as in, a few people order only food out of a personal niche habit, but most will order something to drink).
Accordingly, most German menus I have encountered have an extensive drinks section with various alcoholic, non-alcoholic, hot, and cold beverages. Choosing and trying an interesting-sounding beverage from the drinks list can be given just as much attention as choosing and trying an interesting-sounding dish from the main courses.
This leads me to the following concrete answers to your questions:
It is said that in Germany asking for tap water in a restaurant is rude and looks stingy - examples in here and here.
But I don't like to purchase water there as buying it in a restaurant would likely be too expensive
Well, don't order water. There's a plethora of dishes to choose from, so you wouldn't order something as bland like a buttered slice of bread. Likewise, there's a plethora of beverages to choose from, so you wouldn't order something as bland as a glass of water.
(Caveat: Those Germans who do order water do, in my experience, connect some sense of exclusivity with the presumably particularly high-quality bottled water they will get served.)
So I wonder if it is rude to buy it somewhere like a convenience store or supermarket in advance and bring it to a restaurant.
Even without the probable violation of house rules1: Yes, it is just as rude as buying your food in a convenience store or supermarket and bringing it to the restaurant.
Or is there anything others you can drink in cheaper price while eating?
As I mentioned above, the set of available drinks is not a secret. That is, you don't have to rely on "insider knowledge" to know what you can order besides water. Along with the menu for choosing something to eat, you will get a list of available beverages (either as a part of the regular menu, or as a separate booklet) that will normally list both the price and the volume of liquid you will get. Like this, you can choose something that best meets your requirements for liquid intake and monetary expenses.
1: Note that those are not necessarily very easily visible in Germany. Maybe it is legally required, and maybe they are indeed posted somewhere, but it is usually understood as common sense that you cannot consume anything bought outside in a restaurant in Germany (except due to dietary restrictions, as explained in another answer). However, in some highly-frequented tourism spots in Austria, I have actually seen some restaurants that had very visible warnings signs threatening to charge a fine if someone is spotted consuming food bought elsewhere.
Local here.
Is it rude to bring bottled water in a restaurant in Germany?
I would perceive it as such—it’s a bit like not ordering food and eating sandwiches you brought.
It is said that in Germany asking for tap water in a restaurant is rude and looks stingy - examples in here and here.
That varies greatly. I frequently ask for tap water (I usually order wine as well), and responses vary greatly. Some will happily bring you a caraffe of tap water, in one instance they kindly advised me that they would charge € 1 for it (a rather symbolic price, so I was OK with that), while others would flat-out refuse (which I find rather rude on the part of the restaurant—in fact there recently was a lot of media coverage about a famous German actor who charges € 4.20 for a glass of tap water in his restaurant).
There is even an intiative for serving tap water at public venues, including restaurants.
That said, what else you order makes a big difference. Getting a main course (and maybe some wine) and asking for some tap water is surely a different thing that odering just tap water (which would indeed look stingy).
Bottom line: Asking for tap water is free. If you find yourself in a place that doesn’t offer it, support those that do—local patrons will appreciate it.
I am half German and have lived there for decades. It is generally considered rude or at least stingy but if I have already bought something (even something as cheap as a latte macchiato) I always ask for my water and get it without complaint too. At the time I left Germany (2015) many good restaurants were starting to offer a tiny glass of water with the coffee, just as various Mediterranean nations have been doing it for hundreds of years (with a normal sized one). I just follow their example and ask for a normal sized glass. I even go so far as to ask for water with my wine. It is my right to enjoy my wine and not drink it to quench my thirst.
When the bill comes I give a decent tip to the waiter and with time, if the opportunity presents itself, I make a nice remark. Remember, the waiter cares more about his tip than about selling you table water. By the third time I go, I say "Und...." and the waiter already laughs "ein großes Wässerlein".
I think I am considered poor, but likeable, which is an assessment I can live with.
EDIT:
On the subject of bringing your own stuff I would second others ' assertion that it's socially unaccepted and I haven't tried it. However:
I don't want to advertise anything here, but as this is a site for travelers, it must be noted : one big American coffee chain, which is present in every major German city (last time I checked) had the policy of allowing you to bring everything- be it drink or food. It goes without saying that you won't experience the local flavour in such a place.
First of all: I am a German living in Germany (so please excuse potential language mistakes)
Skip this paragraph and go straight to the summary if you are interested in a quick answer.
A German restaurant visit is always connected to some "procedure":
When you go into a restaurant and sit down with your friends a waiter will come to hand out the menu and take up orders for beverages. Soft drinks, beer, (carbonated or still) water and coffee are the usual choices - you can ask for just about anything though. When asking for water you will probably get high quality water, mostly served in small bottles, not tap water).
Usually everyone orders something individual, however it is also quite common to order a large bottle of water and share it with your companions. This is probably the cheapest option and still leaves room for individual beverages.
Beverages are listed in the menu or in a separate drink menu including prices and volumes so you can choose whatever is most appropriate to your thirst and financial situation - but be quick the waiter will ask for drink orders soon after sitting down!
Then you will have some time to look at the menu while the drinks are prepared.
When the waiter comes back with your drinks he will ask something like 'Wissen Sie schon, was Sie essen wollen?' or 'Was darf ich Ihnen zum Essen bringen?' (meaning: 'have you made up your mind about the food?' / 'What can I bring for food') Then you will order food and once the waiter is gone you and your friends will raise a toast (if you are on a business dinner or in a gallant restaurant you should not have touched your drink until now - with friends this rule is not too strict but decent people will still wait until everyone has his drink).EDIT: "raising a toast" in this case does not necessarily mean holding long speeches - in fact this part usually comes down to a simple touching of glasses and saying "Prost".
You can order more beverages whenever the waiter is close and in a good restaurant you will be asked for another beverage once the waiter notices an empty glass.
When the food comes one waits a short while with the intent to wait until everyone has his food but usually someone will say 'Fang ruhig schon mal an' = 'you can start eating', and if the food is not delivered at once it is OK to ask 'stört es jemanden, wenn ich schon anfange?' = 'do you mind if I start eating?' and nobody will say anything against it.
(skipping dessert since I notice I am going off-topic)
As the end of your visit comes closer you should think about paying your bill. In larger groups it is common to call the waiter and pay your part of the bill. In smaller groups you will usually be asked if you want to pay altogether or separately, but sometimes (especially in cafes) it happens that you receive a printed bill for the table and then pay as a group.
Please understand also that restaurants usually rely on the income of beverages very much since they accommodate the prices and expect you to buy beverages. If you cheat (e.g. bring your own water and secretly refill your glass under the table -> very rude) the restaurant will not make enough profit to cover their expenses.
SUMMARY
Having said that much about the procedure and the financial situation you can see that beverages are very important. And to directly answer the question: You should absolutely NOT bring your own beverages or food. In most restaurants this is actually forbidden by house rules and it will at least be considered very rude! You can order still water or carbonated water but you will not get water from the tap in most restaurants (and in the other restaurants you will probably be frowned at)
There are some excuses from this general agreement:
(Copied last two list bullets from TomTom's answer)
One more piece of advice: Since I am also not too rich I usually drink a glass of water before going to the restaurant and buy a small soft drink or a beer (beer is surprisingly cheap) instead of going there thirsty and ordering many beverages.
Most of what I said was already mentioned in other answers but I tried to sum it all up. (I wanted to make some comments first but I didn't have enough rep)
Disclaimer: I am German and my parents ran various restaurants during most of my life.
The other answers already explain how this rude. I would go one step further and say it is not only rude, but indeed socially unacceptable. Most Germans will not actually dare to do that, because it feels quite shameful to us. This feeling is so strong that I will actually start to panic if an international friend on my table gets their own bottle of water out in a restaurant.
It's also common to be asked to leave drinks that you are already carrying outside the restaurant or even food stall. Restaurants have the prerogative to sell what they want to whom they want. They can easily refuse you service, not let you in or kick you out if you misbehave.
Typically if a waiter sees you drink your own drink, they will politely ask you to refrain from this behavior once. After that, they will probably charge you for the drink. This is entirely legal. It's called Korkgeld. It's meant to replace the loss the restaurant has because you're not buying something from them.
So if you bring a bottle of water, expect to be charged for a bottle of water at their usual rate.
Bringing water might be somewhat rude or even forbidden but it is in any case not the done thing. If anything, asking for tap water makes more sense than bringing a bottle of water. It's not common because Germans like carbonated beverages and frequently drink sweet beverages or even coffee with food but it might still be possible, possibly for a small charge.
There is absolutely no point in bringing your own bottle of water. It's awkward and deprives the restaurant of one of their major sources of profit. If they don't like serving tap water, they won't like you bringing bottles. Besides, it's a very competitive industry and it's not uncommon for the food to be priced very tightly and beverages to have a higher margin. They are not so much “exploiting it” than just trying to break even.
If beverages are too expensive for you, you might consider eating take-away, self-catering or simply not drinking during the meal as alternatives but bringing a bottle of water is not better than requesting tap water.
If you mean "rude" is a synonym for "illegal" - yes. See, restaurants in Germany do not allow you to consume brought beverages and food (with medical exceptions, like baby food or something along the line). By entering a restaurant, you agree to adhere to the rules of the place. By breaking the rules, you break the law.
There are some exceptions:
Being stingy does not.
Want to save money? Do not go to a restaurant. From the perspective of a German, the attitude shown by the query stinks - If I can not afford to even drink a glass of water in a restaurant, I make a picknick - buy food in a supermarket and take it from there.
illegal and braking the law do definitely NOT apply here! The rules prohibiting bringing your own food are house rules not laws (more like terms and conditions of using the establishment). So the worst case thing that could happen is that you are asked to leave.
Hausrecht, which basically says that the premiss owner(?) can define under what conditions others may enter and that he has the right to throw them out if they do not comply. But then again I'm not a lawyer, so if you can point to a specific law covering this I'm willing to be convinced otherwise.
So I'm half German. I live in Switzerland but I eat out in Germany as frequently as I do here since my family lives in Bavaria.
Don't bring an outside drink. DO ask for tap water. At worst you might get a dirty look from the waitron, but dirty looks from wait staff here are so common you won't know if it's because of the water or not.
Asking for tap water is not considered rude. At worst it might be considered a bit eccentric or cheap, but it's not rude. Germany has a pretty lively environmental dialog going on, and many people are aware of the following facts: 1) The energy and pollution costs of tap water are dramatically less than bottled water. 2) The tap water in German and Switzerland (and almost certainly Austria, though I haven't checked) is generally better (healthier, more free from possibly toxic or harmful pollutants) than bottled water. So there's a growing movement to drink more tap water and less bottled water. That fights against a pretty longstanding tradition of bottled mineral water, but tap water is clearly the way to go.
This issue actually made it into the papers and talk-radio a could years ago. The overall consensus was that if the restaurant doesn't want to give you tap water, that's their problem. It's not considered obligatory to give you the water free though, so don't be surprised if you get charged for you tap water (I think that's only ever happened to me once).
Generally tap water will be your cheapest option, followed by the house beer.
Yes, according to my experience (native German), bringing drinks to a restaurant is not only considered rude, you are likely to be asked to put them away, leave or pay a fee (this is not very common, but I have been to restaurants which even list that fee on their menu, basically a "bottle fee" for any drink you bring by yourself).
Asking for tap water is the much preferred option. While I hear that people think it is rude, I have never seen a waiter consider it such. Sure, they might prefer the nice profit they have on drinks, but I've seen tap water ordered many times in german restaurants, and it has always been provided without trouble.
As others have said, do not bring you own.
You can order tap water and they can charge you for it. I don't think it's wrong for them to charge: after all, there is a cost associated with the location, staff, and other business costs. An alternative would be to subsidise the cost associated with serving tap water with other purchases. Although normally, the cost charged by a restaurant may be around 3× the cost of the product if you were to consume it otherwise, that doesn't apply to tap water, where the cost is around 0.2 cent/litre. See a recent article:
Darf Leitungswasser im Restaurant etwas kosten?
In a 2014 sample of 30 restaurants in Vienna, Austria, 27 charged between €0.30 and €3.60 for a glass of tap water. Maybe it's similar in Germany.
(German native)
It is not rude, and it is not inacceptable in any way, but in all likelihood (almost guaranteed) you will have no choice but to buy drinks anyway.
There are exceptions. In Bavaria, you will get free water for your dog (which, maybe to your surprise, you can bring into the restaurant), and as a native, you may get a "Schnitt" after having some beers (half-refill for free, for the way home). Also, in a typical Biergarten, you will be allowed/expected to bring your own food, but will still buy your beer there.
Other than that, you are almost certainly out of luck. You are expected to buy drinks, and buy them you will, or you will not be served.
The "justification" is that in Germany dishes are quite cheap, which is what people want, too. To compensate for this, the poor, starving innkeeper has to make drinks a bit more expensive and cannot give out drinks for free.
Now, the above is a rather obvious lie -- neither are dishes cheap (I pay the same, give or take 5 Euros here as I pay in France, only in France I do get free water), nor do "people" (who, anyway?) want cheap dishes. If anything, most people will want a dish that is worth its price (there's even a German word to distinguish between cheap and worth-the-price). Finding that may however be a challenge in some regions. Other than in some neighbouring countries where the art of opening a bottle or serving a plate is serious business (France, eh?), in Germany it is by no means certain (or even likely) that the people running the restaurant are gastronomy professionals at all ("Wer nichts wird, wird Wirt" -- Approximately: Him who didn't learn anything better, becomes innkeeper).
But, it is what it is, sadly.
Some restaurants have meanwhile adopted the "mediterranean" style where you indeed get a caraffe of tap water with a meal, but they are very, very rare. Maybe, if you are very lucky, there's one such place in a city, but don't count on it.
Alas, as the French would say: If one can't afford, one shall picnic.
Besides laws rules etc., it is surely considered rude. And especially so if by restaurant we refer to somehow valuable (menu quality, service, design, etc.) place. This stated, in Austria I was immediately invited to put back to my bag a beer which I put on the table just because I was reordering my stuff. The manager did not even accept my explanation in spite of me having already more items than a standard Mcdonald menu nicely displayed on the same table. There might be owners of some business who do not care but this is then very case specific. Of the European countries that I have visited, only in UK is non rude and common to bring your own WINE bottle to a restaurant. I imagine that this stop at some high niveau, tough.
Another point is that the world "restaurant" might be ambiguous, at least can leads to potentially false friends interpretation as it is very similar in many languages but not necessarily identical. For instance, given that a restaurant is an establishment serving food, in italian you do not go to restaurant every time you eat in a business. And there are not restaurant billing 3 euros (and even not pizzeria nor fast food, by the way). If your bill is below 10 euro, in practice you took a sandwich, whatever the business is called. Although a place might be technically considered a restaurant or not, residents in the countries mentioned above tell "I have been at mcdonald's / paninoteca / beergarten /... and not" at restaurant " whenever it happens to eat somewhere that is not home.
