The Collaborative International Dictionary of English v.0.48:
Water plant \Wa"ter plant`\ A plant that grows in water; an aquatic plant. [1913 Webster]WordNet (r) 3.0 (2006):
water plant n 1: 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, water plant, hydrophyte, hydrophytic plant]