water lily vs aquatic plant

water lily and aquatic plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
water lily	Yes	No	No	No
aquatic plant	Yes	No	No	No

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

water lily: an aquatic plant of the family Nymphaeaceae
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 water lily include hydrophyte, hydrophytic plant, water plant.

water lily (noun)	aquatic plant (noun)
an aquatic plant of the family Nymphaeaceae	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