It's like a space shooter, except you're underwater and you're forced to play by FPS rules. This could have been a good combination, but I found it to be extremely repetitious and, ultimately, tedious.
It does support full co-op play in the campaign mode over Xbox LIVE, though. It gets bonus points for that. The problem is, the game isn't really worth playing. It tries to be: the different character classes, upgrades, and
factions are a testament to that. The problem is, when you strip all the attempts at being replayable away, you're left with a king of the hill game that gets old real quick.