Undertow is a fairly good looking Xbox Live Arcade game, unfortunately it lacks substance and gets boring very quickly.
There are 3 different factions and 4 different types of unit you can switch to per faction, though they are all fairly generic. The game plays out like an underwater side-scrolling version of Battlefield's Conquest Mode, there are a number of bases that you must capture tug-of-war style on any given map, the object being to run down the opposing side's tickets, the more bases you hold the faster they run down. That's about all there is to it, there's also a campaign mode with cut-scenes but it's not very exciting. All in all it's not a bad arcade game, but it's not a good one either. It just sort of falls into the mediocre area, I got it for free though so no complaints there. But your Microsoft points are better spent elsewhere in my opinion.