I've got some suggestions. So;
How about doors? There could be a door with a window and just a regular door.
There could be birds flying around the ocean, and the player could hunt them, tame them or just simply let them be. The taming would be giving them food, (food as seeds) and maybe helping them.
New weapons, like arrows, you could make a bow or just throw them around (the bow would make more damage but take longer to charge).
Bananas, apples and different berries. Edible and non-edible flowers would be nice too.
Different fish and sharks.
Difficulty levels, they would define how many sharks there are at a time, how much recourses would pass by and how quickly the player would become hungry and thirsty.
Multiplayer. We need multiplayer.
Make the raft look like it's moving more, there could also be a way to lock yourself to one place.
Day-night cycle, seasons and weather.
A way to make stronger rope and put it on the planks. It could be just decorative, or it could be used to tie two areas of the raft together. If it was just flowing in the water with no purpose, the shark could hang onto it and rip the plank it was connected to off.
The more the player would build things, the better they would get. If the player is good at building and has the necessary recourses, they could make some really cool even automated stuff. The systems would not be the best, and the player would still need to get fuel, so it wouldn't become boring.
There could be a gamemode where the player would need to stay alive in extreme situations for a certain amount of time before they get rescued.
A backstory. Why is the player on the raft?
Also, sorry if I made any typos, English is not my native language, forgive me.