marsh plant vs water plant

marsh plant and water plant both are nouns.

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

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

marsh plant: a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath
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 marsh plant include aquatic plant, hydrophyte, hydrophytic plant.

marsh plant (noun)	water plant (noun)
a semiaquatic plant that grows in soft wet land; most are monocots: sedge, sphagnum, grasses, cattails, etc; possibly heath	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