Source : Webster's Revised Unabridged Dictionary (1913)

Water plant \Wa"ter plant`\
   A plant that grows in water; an aquatic plant.

Source : WordNet®

water plant
     n : a plant that grows partly or wholly in water whether rooted
         in the mud, as a lotus, or floating without anchorage, as
         the water hyacinth [syn: {aquatic plant}, {hydrophyte}, {hydrophytic
         plant}]