The only free and comprehensive online etymological dictionary of the Spanish language
agua f. (Noun) "water"
10th cent. From Vulgar Latin acqua 'id.' From Latin aqua 'id.' From Proto-Italic *akwā- 'id.' From Proto-Indo-European *h2ékw-eh2 'id.' Connected to *h2ep- "water," but it is impossible to know whether *h2ekw- or *h2ep- is original. Transitioning between /kw/ and /p/ is a common sound change.
As a surname Agua, it was applied to families located near founts and other sources of water.
Indo-European
Romance
Asturian agua, Portuguese água, Galician auga, Catalan aigua, Occitan aiga, French eau (Old French ewe, egua), Italian acqua, Aromanian apã, Romanian apă, Megleno ápu, Sardinian abba.
Germanic
Gothic aƕa "river," Old Norse á 'id.,' Old Saxon aha "water," Old High German aha 'id.,' Old English ea "stream"

In the 3rd or 4th cent. text Appendix Probi we find the line: aqua non acqua "[the word for 'water' is] aqua, not acqua." The author's spelling correction proves how the word was pronounced by common speakers, and offers a rare glimpse of the evolution of Latin into Romance languages such as Spanish.

Western Romance and Sardinian cognates show a late voicing of Latin -qu- while Eastern Romance does not. The evolution from Latin -qu- to Romanian -p- is an entirely normal change (cf. Latin equa > Romanian iepã "mare"). Sardinian abba is a normal product of /kw/ in Latin (cf. Latin linguam "tongue" > Sardinian limba).

In the medieval period, water flavored by flowers took a variety of names (agua de angeles, agua rosada, agua de azabar, etc...). Water was also employed as a euphemism for alcoholic beverages (e.g., agua ardiente "brandy").