If you think about it, Sins could make a pretty snappy under-water RTS mod. Think about it; massive water planet, fighting for 'floating' cities in the middle-depth of the ocean. It wouldn't be too hard, and would add some varatity to most of the mods, which will likely be based in space.
Add some fins and fans to the ship, and some 'leaking' bubbles would give a nice impression. Ships would 'hyperspace' when it's safe to (far enough from water debrey). Major resources could be Oxygen, and (obviously) some sort of building material. I haven't really thought alot about the general ideas that could be done, but if I had the time and I was making a mod for this game; it would be definetaly underwater. (Sorry for any spelling mistakes, too lazy to put into Microsoft word today)