water chestnut vs water plant

water chestnut and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
water chestnut	Yes	No	No	No
water plant	Yes	No	No	No

As nouns, water plant is a hypernym of water chestnut; that is, water plant is a word with a broader meaning than water chestnut:

water chestnut: a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits
water plant: 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

Other hypernyms of water chestnut include aquatic plant, hydrophyte, hydrophytic plant.

water chestnut (noun)	water plant (noun)
edible bulbous tuber of a Chinese marsh plant	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
Chinese sedge yielding edible bulb-shaped tubers
a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits