Some of these are more advanced, but I stuck to what I knew was doable without any massive amounts of money invested, such as multiplayer.
1. Raft/Canoe... may be used to travel around the sea a bit more, and reach loot in the ocean.
1. Buoys... Some stationary buoy's located around the ocean, that you may sail to using the raft. Some more advanced loot may be placed there for looting.
3. Until some ideas for further implementation of the world border come up, I think having a world loop (if you go over the side on one side, it puts you on the side adjacent to it (opposite side)) may be interesting
4. Snorkel... pretty self explanatory, you may make a snorkel (basic) out of wood and thatch or something, and then simply be on the surface of the water, or a snorkel (advanced) to go down deeper.
5. Ocean caves or ravines, accessible using the snorkel. May contain a dungeon or something maybe?
6. Advanced materials, such as clay, sand, etc. For example, you may use sand to make glass.
7. Windowed wall tiles, so that the wall has a window on it. My apologies if this is already implemented by the time that I write this.