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 practice, that mixes a few foocubi into the summons. There is a 1⁄260 chance, however, that they may summon Juiblex or Yeenoghu.
A player may not polymorph into a water demon.
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)%, 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.
In 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.
[ The monkey king ] walked along the bank, around the pond.
He examined the footprints of the animals that had gone into
the water, and saw that none came out again! So he realized
this pond must be possessed by a water demon. He said to the
80,000 monkeys, "This pond is possessed by a water demon. Do
not let anybody go into it."
After a little while, the water demon saw that none of the
monkeys went into the water to drink. So he rose out of the
middle of the pond, taking the shape of a frightening monster.
He had a big blue belly, a white face with bulging green eyes,
and red claws and feet. He said, "Why are you just sitting
around? Come into the pond and drink at once!"
The monkey king said to the horrible monster, "Are you the
water demon who owns this pond?" "Yes, I am," said he. "Do
you eat whoever goes into the water?" asked the king. "Yes,
I do," he answered, "including even birds. I eat them all.
And when you are forced by your thirst to come into the pond
and drink, I will enjoy eating you, the biggest monkey, most
of all!" He grinned, and saliva dripped down his hairy chin.