We've heard very little about the underwater areas in Guild Wars 2 but nothing substantial until recently. When talking about underwater segments, ArenaNet wasn't referring to swimming from point A to B like most MMOs, but actually taking the gameplay down below, against a variety of unique undersea creatures.
When underwater, players have total freedom of movement, without any environmental hindrances like movement penalties or breath meters. Additionally, your skill bar also changes to a different set, exclusively for use in underwater regions, and these will vary per class and weapon.
A recent preview from PC Gamer dropped an Engineer in the water. Soon as you step in, your action bars are consumed by bubbles and replaced by the new set. Your weapons may also change, as only certain weapons can be used underwater; for the Engineer, that's the speargun. Most land-based skills will have an underwater equivalent, though it's understandable if you don't get quite as many at your disposal while fighting mutant fish. The "Oil Slick" skill, for instance, leaves a trail of oil on land to blind enemies, while underwater, it becomes a cloud with the same blinding effect on nearby enemies. The preview also mentions Elementalists being able to trap enemies in air bubbles, causing them to float helplessly up to the water's surface.
We'll no doubt hear (and see) more as 2012 approaches. For now, enjoy the new underwater-themed screenshots.