Shemeska said:*blink* Wait... what in Gehenna is a 'water demon' in the first place? That's got to be some sort of goofy placeholder. Is there actually a 'water demon' in some source I'm apparently not familiar with?
Dragon #306 had an article called By Evil Bound by Mike Mearls. It included six non-tanar'ri demons, six yugoloth creations, and six non-baatezu devils.
The elemental demons were supposed to be the raw material of the Abyss animated by patchwork souls left behind from the remnants of a great Blood War battle. They included air demons, ash demons, earth demons, fire demons, ice demons, and water demons. As I said, they weren't tanar'ri: they were more like Abyssal mephits.