An Etymological Dictionary of the German Language/Annotated/Wasser
Wasser, neuter, ‘water,’ from the equivalent Middle High German waȥȥer, Old High German waȥȥar; compare Old Saxon watar, Dutch water, Anglo-Saxon wœter, English water; beside these West Teutonic forms in r (watar-o-) are found the forms in n, Gothic watô, Old Icelandic vatn, neuter, ‘water.’ The root wat is related by gradation to ut in Otter, and to wêt in Anglo-Saxon wœ̂t, English wet, North Frisian wiat, ‘damp, wet.’ The other Aryan languages have also corresponding graded forms with the same signification; Aryan ud in Greek ὕδωρ (Latin unda?), Old Slovenian voda, Sanscrit udán, ‘water, billow,’ udrín, ‘abounding in water, and also the root ud, ‘to moisten’ (with which the cognates of Modern High German waschen are connected?). Hence the Aryan root is ū̆d, wē̆d, wō̆d. The Aryan word corresponding to Latin aqua assumed in Teutonic (Gothic ahwa) the meaning ‘river.’ See Au.