maybe we can go underwater for a certain amount of time?
day/night cycle would be cool.
different sharks or other things. some friendly some not.
fires, tents, maybe you can add something that can move our rafts, but the shark will always follow you.
small islands near by that have some loot.
crabs in those islands?
different fish.
you could add ship wrecks that you could explore in.
and one more thing...MULTIPLAYER!!!!!!!!!!!