Only feasible concept for underwater I can figure would be enclosed underwater facilities, with no access to the outside (and the outside would only be there, really, to say "Hey! Look! I'm an underwater level! Look at me!"). Again, though, unless someone can come up with a damn good explanation for why such an isolated underwater facility would even be considered a priority target by the aliens, it is nothing more than an artistic exercise, and mildly resource-inefficient at that.
Perhaps for multiplayer, maybe.