Breathing equipment for characters, their underwater skills and weapons are all equipped automatically. It's a "vital, important part" of the whole game.
Oxygen is only a factor when your character is drowning after having been defeated, i.e. your health has reached zero. At this point, much like on the surface, you can be revived by an ally or kill an enemy to bring yourself back from the brink.
As not all skills are compatible with an aquatic environment a set of underwater abilities and weapons are automatically switched on and off. Also you'll be fighting in a 3D space unlike on land so spells and the like can have a full sphere of influence.
Apparently we can also pull enemies up to the surface to defeat them more easily or even drag land lubbers down. "We’re not kidding when we say that the underwater experience in Guild Wars 2 is going to be a vital, important part of the game," said Guild Wars 2 systems designer Jonathan Sharp, via the .
"You can’t experience everything the game has to offer without getting wet, so grab a breathing device and a trident and hit the beach! Life is much better, down where it’s wetter, under the sea." There is still no official release date for Guild Wars 2.
Check out thefrom ArenaNet's Jonathan Sharp.