The best Xbox Live Arcade Game there is. Don't miss out, the $10 price tag is worth the fun. Undertow is awesome.
There are three campaigns, one for each of the races featured in the story. There are the Iron Marines (underwater English Mercanaries), the Nemonians (led by the insane Captain Nemo) and the Atlantians, an undersea people who fight with magic. The story is told suprisingly well for something that isn't considered a "real game" but believe me, this should be.
The objectives of the campaign missions and the online games are to kill the enemy while fighting for control points on the map. If the enemy gets all of your control points, you lose. The game does a good job of making you choose whether to attack or defend and overall the gameplay is purely enjoyable.
The game plays well, using a left stick to move, right stick to shoot interface. There are depth charges (read: grenades) that you deploy which are exremely fun to use. There are the standard movement features (boosting, turbo) but what impressed me the most was just how easily and well the characters move. They move like people underwater should.
The game also features multiplayer support for up to 16 people. The online plays just like the single player does except you get the joy of knowing someone may have thrown their controller after you planted a depth charge juuuust riight.
If you're looking for a good game to buy for $10, this is it. If you aren't into this kind of game, at least try the demo, you might just want to get it.