Water

Water comes in several forms:
 * as a potion of water
 * in a fountain
 * in a sink
 * in a pool or moat

Water is that clear liquid that you probably know from real life. In NetHack, water can for example rust objects or drown you. In a fountain or as a potion of holy water or unholy water, water has more magical effects. In a sink, water tends to be polluted, and it can have bizarre effects; for example, it might mutate (polymorph) you when you drink it.