First thing to say is congrats by the game, is a excellent game anyway.
I saw a lot of good sugestions like weather, storms, waves, different kind of fishes and sharks, bed, temperatures, day and night cicles ....
But you can think about underwater. A possibility to craft some kind of system to breath underwater to dive and explore and collect resources such as stones, wood and metal from shipwrecks.
In this case the shark is less predictable and have how to defend ourself from him diving.
Thanks.
Chris.