water lily vs water plant

water lily and water plant both are nouns.

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

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

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

water lily (noun)	water 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