Ok hear me out: A region with caves and a tide system depending of the time of the day, players have access to the caves even when flooded but they have to watch out to not drown.
Ship could be stranded in shallow water if they get too close so the players would need rowboats…
Ideas for new islands
I like that concept, need to tie it to a little bit more stuff too. Alone I don't think it sticks out enough. How about stalactites and stalagmites that can fall from the ceiling and stick out the water to really make it difficult to navigate into the caves. You could also have extremely tight areas where you have to inch through to get in, would take extensive use of your pad/mouse to assess through.