waterweed vs aquatic plant

waterweed and aquatic plant both are nouns.

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

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

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

waterweed (noun)	aquatic 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