duuuude, pressure is consistent throughout the Elemental Plane of Water so there's no nitrogen narcosis and you can go EVERYWHERE. I mean like the only thing that doesn't make the Elemental Plane of Water the awesomest place in D&D cosmology is that because there's no surface there are no cetaceans or pinnipeds.
@Leucrotta I think 3rd Ed Manual of the Planes mentioning occasional air bubbles as you get closer to the Plane of Air.