hydrophyte

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A plant adapted to grow in water.

from The Century Dictionary.

• noun A plant which grows in water; an aquatic plant.
• noun In phytogeography, a plant adapted to live under conditions of abundant moisture, or, in late usage, abundant physiological moisture. Compare mesophyte and xerophyte.

from the GNU version of the Collaborative International Dictionary of English.

• noun An aquatic plant; an alga.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany A plant that lives in or requires an abundance of water, usually excluding seaweed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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