i have an idea to add more mobs other than the shark.for example there could be seagulls who would steal your food.there could also be crabs which would slowly damage the raft and could also be cooked.
another idea is to add rain and other weather,from which some harsh ones will damage the raft and could make the player sick or something like that.
A cut scene could be shown when a new game is started showing faint memories of being on a ship and seeing it get wreaked and then waking up on the raft with stuff to craft a hook rather than a hook already.
The ability to get some materials back when a building material is broken...(hope that made sense)