Ok, I would like to create a map that's based underwater (like Mav's Rebel Spec Ops Mission 1 i think it was) where units don't drown and they can float around, have any ideas on how I could do this?

Well if I were you I would not actually put water; I would make the world's effects seem like underwater (little particles floating around, darker and greyer) and at the top of the level have a big flat model that looks like the surface of the water(though I can't think of a way to make it move like water other than scrolling, which I have no idea how to do because Im really bad at modeling and hex editing and that kind of thing) and for people swimming you could give them infinite jet packs with no hud like darth vader to simulate swimming.

ok ty