I think you could add the next element:
- more item to craft with metal
- new item who need a lot of material for craft
- a main objective (like getting back to dry land)
- a way to lure the shark for fish it
- a delay between the kill on the respawn of the shark
- a way to craft an underwater building
- an alert when the sark attack (Visual and audible)
- a way to immune the boat to shark
- recover some material when you destroy item
- add some rare material that can be gotten by new periodical event:
- pirate attacks
- small island found
- an iceberg / Submerged rock enconter
sorry for my english, it's not my native language.