Normally when you go underwater in a game, you have a breath meter that determines how long you can stay down there. This helps level designers design their oceans by limiting how far down the player can go. After all, not all games are Endless Oceans. It also keeps your character’s fingers from pruning.
Guild Wars 2, however, has some pretty wide ranging underwater exploration. When the level designers need to keep you from just heading out into the empty seas, they cite “currents” as the limiting factor. I would have also accepted “undertow”. But when it comes to briny depths, there’s some awfully amazing stuff in the water. And since all characters have rebreathers with unlimited air capacity and magical defenses against nitrogen narcosis and the bends, you can explore freely! The only drawback is that you look like a certain ridiculous villain in a certain recent movie.