The only free and comprehensive online etymological dictionary of the Spanish language
agua f. (Noun) "water"

10th cent. 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. Changing from /kw/ and /p/ is common and even occured in Eastern Romance languages.

As a surname Agua, it was applied to families located near founts and other sources of water.

Romance: Western Vulgar Latin: Asturian agua, Portuguese água, Galician auga, Catalan aigua, Occitan aiga, French eau (Old French ewe, egua), Italian acqua; Eastern Vulgar Latin: Aromanian apã, Romanian apă, Megleno ápu; Sardinian: abba.

Indo-European: Germanic *ahwō-: Gothic aƕa "river," Old Norse á 'id.,' Old Saxon aha "water," Old High German aha 'id.,' Old English ea "stream"

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").