This is pretty fun for about an hour, but I very quickly found myself with too many supplies and not much to build or craft. After I figured out how to start without dying instantly, I set myself up with a collection of item nets, purifier and cooker, and I was comfortable in no time. I could really use some goals. I like the island idea that's been suggested, but at the same time I wouldn't make much sense if the point is to live on a raft.
I think NPCs would be a good plan. Perhaps some other lost-at-sea characters could drift by, and you'd need to be able to provide for them as well. You could even assign a job to the NPC, so that while you focus on fishing and cooking, they repair damages, or fight off the shark. They should be rare, though. Even rarer could be a stranded animal, like a dog, which could help attack the shark, and randomly pick up items that pass beside or under the raft (probably mostly planks). Again, you'd have to feed it. The NPC/dog would also have thirst and hunger, as well as a health bar, so you'd have to make sure they didn't get pushed into the water, or could at least get out if they did. Dog would have to come with a bowl, which you could fill with clean water. Feeding could either also be done with a bowl, or just by selecting food and pressing E at the dog. The NPC could either get their own food and water, given it's available on the cooker, or you might have to hand it to them.
I feel like this would add a challenge, and in some cases force you to decide if you can afford to care for another life in exchange for a benefit or two.
Multiplayer also sounds like an interesting idea, as long as you don't add the option to fight other players.
More animals would also be a fantastic addition! Birds and other sealife that you could stab to eat would be really nice. Turtles, dolphins, even barracuda. Having dolphins could offer a sense of paradise or peace, but could also, if you're desperate, become food. The shark could also target them, giving you the chance to swim out for supplies, OR you could try to save the dolphin.