naiad (n.)

"water nymph," c. 1600, from Latin Nais, Naias (genitive naiadis), from Greek Naias (plural Naiades) "river nymph," from naiein "to flow," from PIE *naw-yo-, suffixed form of *(s)nau- "to swim, flow, let flow" (from PIE root *sna- "to swim"). Dryden used the Latin singular form Nais, and the plural Naiades is attested in English from late 14c.