Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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