water pimpernel vs water plant

water pimpernel and water plant both are nouns.

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

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

water pimpernel: a white-flowered aquatic plant of the genus Samolus
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 pimpernel include aquatic plant, hydrophyte, hydrophytic plant.

water pimpernel (noun)	water plant (noun)
a white-flowered aquatic plant of the genus Samolus	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