'water-related' originates from Modern English, specifically the compound of the noun 'water' and the adjective 'related', where 'water' meant 'water' and 'related' meant 'having a relation to (something)'.

'related' comes from Latin 'relatus' (past participle of 'referre') via Old French/Medieval Latin into Middle English as 'related/relaten'; 'water' comes from Old English 'wæter', from Proto-Germanic '*watar' and ultimately from Proto-Indo-European *wódr̥. The modern compound 'water-related' was formed in Modern English by combining these elements.

Initially the elements simply meant 'water' and 'having a relation to'; when combined as 'water-related' it has kept the straightforward meaning of 'connected with or caused by water'.