water buttercup vs water plant

water buttercup and water plant both are nouns.

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

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

water buttercup: plant of ponds and slow streams having submerged and floating leaves and white flowers; Europe and North America
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 buttercup include aquatic plant, hydrophyte, hydrophytic plant.

water buttercup (noun)	water plant (noun)
plant of ponds and slow streams having submerged and floating leaves and white flowers; Europe and North America	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