water lettuce vs water plant

water lettuce and water plant both are nouns.

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

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

water lettuce: pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes
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 lettuce include aquatic plant, hydrophyte, hydrophytic plant.

water lettuce (noun)	water plant (noun)
pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes	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