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 to 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 hostile demon will attack. A friendly demon grants a wish. A demon will be hostile with probability (80+DL)/100, where DL is your current dungeon level. 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.