[Wids]

The elemental plane of water isn't flagged as an underwater area, therefore water breathing helmets or spells aren't necessary there.

Wow....so that's a bug I never knew existed. Please plan for the elemental water areas to require the ability to breathe underwater as well sometime in the very near future.

Oh wow, I always thought that was deliberate.

...kind of like how the Elemental Plane of Fire doesn't promptly reduce you to a cinder upon entering it, or how you don't immediately suffocate on two lungfuls of topsoil when you enter the Elemental Plane of Earth. If D&D 2nd Edition is to be believed, I figured that it was because the wormhole magic which allows passage to other planes also adapts the interplanar travelers to survive in those planes.

(Of course, IIRC, 2nd Edition addressed that problem by physically converting the adventurers into elementals native to the Elemental Plane in question, along with their equipment; after all, only earth in its various forms -- including solid, liquid and gaseous states of earth -- can exist in the Elemental Plane of Earth, and nothing else can. Yes, there's actually solid air to walk on in the Elemental Plane of Air. That's elemental magic for you.)

Of course, whether Thay's Elemental Plane of Water is breathable or not, it's still a good idea to enter the plane with water-breathing effects in play, if only to avoid the chance of unavoidable death once the water elementals start throwing pieces of themselves at your nostrils.