How about an update where you can scuba dive with the right gear, of course. So we can discover more things like different sea creatures or an underwater abandoned cave to explore. You can add minerals to the cave and we can mine it to build more sturdy or rare items. You can also have like a rope to tie your raft to something so that it doesn't float away when you scuba dive. Just a thought~ great game btw!!!
Scuba seems unrealistic, given how much gear is involved, but snorkeling would work. Limited range (and your raft keeps drifting) and depth but still an opportunity to explore a bit while still keeping the world fairly small. Perhaps craft flippers to extend the range, add spear fishing (attracts sharks?) or perhaps clams/oysters that can be collected.