What are the seaworthy boats available for a party of 4-6 in Pathfinder, without hiring additional crew?
I put some stats together to model the viking knarr Skuldelev 3, averaging the longship and the rowboat, and using the old AD&D price of 3,000gp. There's also the third form of the folding boat. Are there any others in this class?
Most ship types aren't exactly fixed size, most are categorized by sail rigging. Build a larger ship, have more sail and storage, need more crew. But it's still the same type of ship. So for example saying a sloop will need 4 people and cost 3000 gp will be a very stereotypical simplification. There will be smaller and larger sloops.
A lot of ships can be used with a crew of 4-6. Most Sloops, Caravels, Cutters and Feluccas would match the setting and the amount of sailors. Note that the characters should have some experience sailing.
Pathfinder as well as the D&Ds were always somewhat short of rules on boats and ships. For pathfinder there is a book called Admiral of the high seas that has a lot of ships, descriptions and prices. I haven't used it for Pathfinder as it contains rules for both Pathfinder and D&D, but it works well for D&D, so I guess it should be good for Pathfinder, too.
For example, a cutter, with a full crew of 4 and a minimum crew of 1 costs 1000gp in said book. To man this perfectly at all times, you'd need 10 people, so a crew of 6 seems reasonable if you don't expect fierce sea-battles or raging storms.
If you're going to be delving into high-seas stuff, boats, boat combat, and things of that nature I recommend The Skull and Shackles Players guide. It contains all the information to answer your question as well as additional rules about pirates, peg legs, scars, many different kinds of boats, ship combat, and what have you.
To answer your original question however, I'd say that A caravel or a cutter would probably be your best bet for a group of small people looking to get out on the seas to do some sailing. If you're wanting a step up from those to something with a larger crew that's when you start getting into Carrock and Galleon territory.
Sticking to the default ships included in Pathfinder, there really no great fits, but there are a few that should serve your purpose.
After removing the ones which need large dedicated crews, we are left with just three to consider.
Additionally, The Sailing Ship describes a wide range of ship types, some of which should be sail-able within the bounds you mention. But unfortunately, no examples anywhere near what you are looking for are given. You can always talk to your DM about allowing you to use smaller crews, for reduced speed and responsiveness, but arguably when we get into the full vessels you would need to be a trained sailor to even operate them to begin with.
