Undertow

Based in an alternate world where earth has been completely flooded and several factions are vying for supremacy, Undertow is an underwater shooter featuring a complex multiplayer component. Undertow features a complete campaign mode that can be played either solo or co-operatively with a friend, and also boasts a battle mode that supports up to 16 players at once in an online contest. The goal is to destroy your enemies and take control of their bases, with victory coming from a traditional flag-and-ticket system found in online conquest modes, but the gameplay here is faster and easier to play than other online titles thanks to the simple dual-analog control scheme.

Each faction in battle has its own look and feel -- the Atlantians are a magical race that fight by riding creatures and attack with magic, the Nemodians are a lost and once peaceful race living under the sea for over a hundred years and led by Captain Nemo, and the Iron Marines are a militant group of humans that took to the seas when the world flooded. None of these groups necessarily wants this battle, but with the planet in ruins, civilization is not an option.