waterweed vs water plant

waterweed and water plant both are nouns.

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

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

waterweed: a weedy aquatic plant of genus Elodea
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 waterweed include aquatic plant, hydrophyte, hydrophytic plant.

waterweed (noun)	water plant (noun)
a weedy aquatic plant of genus Elodea	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