Sounds impractical at best.
These humans use sharks for a number of things, specifically to hunt for food.
Call me Mr. Obvious, but if we've domesticated sharks and we need food, what's wrong with shark meat ? :-)
Hunters ride on the backs of sharks
A human "riding" a shark is essentially the shark carrying a packed lunch with them.
This sounds impractical. Sharks are fish and they need to be under the water to breath - they can't air breath and so they can't skim the surface in the way "riding" a shark would require. On the other hand people can't breath water, so there's no happy meeting ground here (unless it's inside a shark's digestive system).
Also note that in terms of stability, sharks (and all sea going creatures) are designed to operate without some dead weight on top of it causing enormous drag in the water. People riding sea creatures may sound nice in children's books, but it's an insane idea unless your idea of a good time is giant wave surfing. It's doubtful a shark would be at all keen on dragging a human around, given it can hunt fine on it's own.
The idea of a shark ignoring food and towing it is just not an option. They kill, they eat. They have no equivalent of "home" : home is where a shark is swimming. A land animal typically will have a location that equates to a home (a base, a lair, a nest, if you will). That's practical on land.
I have had dogs become extinct due to some virus because they suck and are boring. Sharks are much cooler, so I had them take their place as loyal servants to humanity.
First. Things. First. Dogs Are Cool. I'd prefer most dogs to e.g. most lawyers. Lawyers, as an example, are indeed boring, whereas dogs are fun. I rarely have arguments with dogs, but I have arguments with lawyers all the time. Sharks may be your idea of fun, but I can't make that connection myself. I'll stick with dogs. :-)
Humans are land based creatures. Even if we could somehow use sharks in hunting at sea (what for ?), we're going to live on land and we're going to use some other land based creates as our loyal servants (actually dogs are domestically loyal companions not servants). Horses would be the next most likely "servant" (companion). We've used elephants, donkeys, cows, horses and other animals for labor and typically provide them with something in return (like food - not such a good idea with a shark when we're the food !).
But they key here is that we can communicate with these animals is a practical sense and we can provide them something they want. Can't do that with sharks.
Ocean life is far more diverse, with many larger predators
These humans use sharks for a number of things, specifically to hunt for food.
Humans have been successfully hunting marine creatures for food for as long as there are records. We've hunted, on an industrial scale, everything from tiny fish and shell fish to the largest whales. We even hunt sharks !
We don't need help hunting marine creatures. We have evolved a device for doing all that is required ourselves - the brain. That and the ability to create tools, like boats and trawl nets give us all the advantages we require.
How can I make this vision possible?
Not with sharks as we know them.
You might manage something with dolphins. Not sharks.
But this idea (riding a sea creature) is extremely inefficient and unsafe.
Your intelligent creatures will build boats. Now the relative lack of land may make wood a scarcity, but your humans will presumably manage to figure out how to make alternative materials work. Hey, we build boats from metal and we've even built floating objects from concrete. What would you get on a shark for ?
Island cultures need reliable, dependable and predictable transport based on ships of some form to operate a trading system and allow for the exchange of people and idea. Sharks can't do this, and that means once someone developed a viable sea craft, it's game over for sharks and we can go back to eating sharks !
There is too much at stake for an island culture to depend on sharks for anything, except a meal or something we can make from their carcasses.
Humans needing help hunting ? We the ones that make other species extinct !