It would be cool to realize:
1) Scuba dive that would gather resources from the bottom.
2) Whatever the structure of the raft or occasionally swim to the islands, where they could get up to anchor.
3) Add a metal plate in which uvelichina strength up to 1000 for example.
4) Add larger craft, much more.
5) Add a boat, where you can swim to the islands for example.
6) Add a spear to hunt sharks, add more predatory fish.
7) Add multiplayer.
8) Add the possibility of crafting clothing or protection from biting fish.