As any aquarium keeper knows, aquariums or perhaps even a sea (underwater portion, that is) provide a "battle ground" all their own.
My rough idea is this.
Instead of a country, each player would be a species. You could either have straight predators who compete against each other (sharks, groupers, etc.) and various "prey" species spread around. When you "eat" all the species, and/or kill your competition, you win.
OR , to draw on some of the more complicated maps that have recently emerged, each species could have slightly different characteristics. For example (note this is very rough, I am just feeling out interest right now), Groupers & sharks might reproduce slowly (fewer reinforcements), but attack voraciously ( can attack multiple small fish at same time). Anchovies might reproduce very rapidly, but have limited ability to attack. There could even be coral that attacks heavily, moves only very slowly, but is very difficult to take over. ETC (again, the details need to be ironed out better).
I can dig up some nice pictures to use as a start, but will need help from there on. But first, does this interest anyone. Would you play this? Be willing to help me?