Given that there quite a lot of elements (terrain, enemies, etc.) linked to the theme of sea, is there any chance that you expand on the water at some point?
Few suggestions:
- bridges (natural like rock arches or artifical in wood / stones) to connect different near islands
- a harbour building, fishes for ressources, fishing boats and boats transporting units
- war boats
- more "water" enemies like pirate boats, pirates, vikings, kraken, etc.