This game is off to a really amazing start! It's not really like any other survival game I've played, and the only thing I would have against exploration of any sort is that it would make it more similar to Stranded Deep and/or Salt. Having to stay on the raft is the whole point of this game, and I really like that limitation. However, as it is right now, sustainability is reached fairly quickly, and the only reason to build the raft up/out is to have a place to put your stuff, and for appearances. Without a weather system, there's no reason to build sheltered areas. Without food spoilage, it's fairly easy to just hoard veggies and fish. I would like to see some means of deterring the shark, at least temporarily; other people have suggested foundations made of metal, or iron spikes put around the edges, and I like both of those ideas. The player could also leave caught fish tied in the water at the edges of the raft, to give the shark something more interesting than busting up the raft for no reason.
But even with all of those ideas, there's still not an end goal beyond simply surviving as long as possible. Even a "you survived X days" thing like in Don't Starve would be better. Maybe have islands, but since there's no way to steer the raft, have it so that not only are you risking a shark attack by leaving the raft to swim to the island, but put a timer on-screen for the few minutes you have to gather stuff from the island and get back to the raft before it drifts too far out of range to get back safely. Or maybe each day there's a microscopic chance of drifting into sight of a rescue ship or the mainland, like a 0.5-1% chance, just so there IS an end to survive towards. It wouldn't even have to be terribly detailed, maybe the land or ship fades into view and then the game fades to black with "you survived X days and were rescued".