A water demon is a type of demon that appears in the game, and one of the less powerful ones.
Like most major demons, water demons may summon other demons when they attack. At least 5/6 of the summons will be other water demons. All chaotic demons are also eligible, but those whose difficulty is greater than the current dungeon level are exponentially unlikely. In practise, that mixes a few foocubi into the summons. There is a 1/260 chance, however, that they may summon Juiblex or Yeenoghu.
In the early stages of the game, a common way to summon a water demon is by quaffing or dipping from a fountain. These demons may either be hostile or friendly; a friendly demon grants a wish. A demon will be hostile with probability (80+LD)/100, where LD is your current level difficulty; thus, if you have the Amulet of Yendor, a demon will always be hostile. Although water demons are relatively weak for demons, they are likely to wipe out most early-game characters. Players who cannot kill them quickly will probably find themselves fighting two or more.
A player may not polymorph into a water demon.
At the early game, don't quaff from fountains and try to keep use of fountains (dipping) to a minimum. Later on, when you're capable of easily taking on water demons and water mocassins and are carrying several escape items, you can dip items repeatedly in all the fountains from DLvl 1 to 19 for the wish, including in the mines. (This is somewhat safer than quaffing). The chances are very roughly 1/3 that you'll get a wish from some water demon.