'water-friendly' originates from modern English, a compound of 'water' and 'friendly' — 'water' from Old English 'wæter' and 'friendly' from 'friend' + the adjectival suffix '-ly'.

'water' comes from Old English 'wæter' (related to German 'Wasser'), and 'friendly' developed from Old English 'frēond' plus the suffix '-ly'; the compound 'water-friendly' is a recent productive formation in modern English combining these elements to describe compatibility with water.

Originally each element meant 'water' and 'having the qualities of a friend'; combined in modern usage it came to mean 'having qualities that are favorable or not harmful to water or aquatic environments', a more technical/environmental sense than the literal 'friend of water'.