The Collaborative International Dictionary of English v.0.48:

Water nymph \Wa"ter nymph`\
   1. (Myth.) A goddess of any stream or other body of water,
      whether one of the Naiads, Nereids, or Oceanides.
      [1913 Webster]

   2. (Bot.) A water lily (Nymphaea).
      [1913 Webster]

WordNet (r) 3.0 (2006):

water nymph
    n 1: submerged aquatic plant having narrow leaves and small
         flowers; of fresh or brackish water [syn: naiad, water
         nymph]
    2: a water lily having large leaves and showy fragrant flowers
       that float on the water; of temperate and tropical regions
       [syn: water nymph, fragrant water lily, pond lily,
       Nymphaea odorata]
    3: (Greek mythology) any nymph of the water
    4: a fairy that inhabits water [syn: water sprite, water
       nymph, water spirit]