Old English wæter, from Proto-Germanic *watar (cf. Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, Old Norse vatn, Gothic wato "water"), from PIE *wodor/*wedor/*uder-, from root *wed- (cf. Hittite watar, Sanskrit udrah, Greek hydor, Old Church Slavonic and Russian voda, Lithuanian vanduo, Old Prussian wundan, Gaelic uisge "water;" Latin unda "wave").
Linguists believe PIE had two root words for water: *ap- and *wed-. The first (preserved in Sanskrit apah) was "animate," referring to water as a living force; the latter referred to it as an inanimate substance. The same probably was true of fire (n.).
To keep (one's) head above water in the figurative sense is recorded from 1742. Water cooler is recorded from 1846; water polo from 1884; water torture from 1928. First record of water-closet is from 1755. Water-ice as a confection is from 1818. Watering-place is mid-15c., of animals, 1757, of persons. Water-lily first attested 1540s.
Old English wæterian (see water (n.1)). Meaning "to dilute" is attested from late 14c.; now usually as water down (1850). To make water "urinate" is recorded from early 15c. Related: Watered; watering.
measure of quality of a diamond, c.1600, from water (n.1), perhaps as a translation of Arabic ma' "water," which also is used in the sense "lustre, splendor."