One Really good idea would be upgrades like the floor of your raft can go from wood to stone to metal to concrete to titanium and to a point of where there may be an un-destroyable (by the shark) upgrade, but you would still need to balance the cost of some of these to were an unbreakable floor would cost like 100 of all materials, please also add durability to tools so there is a point of making multiple items. One last thing, Armour to make it easier to survive against the shark, and with that accessories with various bonuses (e.g increased stats like food water and health).
-TehMajestickStick