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

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