The Collaborative International Dictionary of English v.0.48:

Water sprite \Wa"ter sprite`\
   A sprite, or spirit, imagined as inhabiting the water. --J.
   R. Drake.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water sprite
    n 1: aquatic fern of tropical America often used in aquariums
         [syn: floating fern, water sprite, Ceratopteris
         pteridioides]
    2: a fairy that inhabits water [syn: water sprite, water
       nymph, water spirit]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "water sprite":
   Davy, Davy Jones, Neptune, Nereid, Oceanid, Oceanus, Poseidon,
   Thetis, Triton, fresh-water nymph, kelpie, limniad, man fish,
   mermaid, merman, naiad, nix, nixie, ocean nymph, sea nymph,
   sea-maid, sea-maiden, siren, undine, water god, water spirit