water lettuce vs aquatic plant

water lettuce and aquatic plant both are nouns.

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

As nouns, aquatic plant is a hypernym of water lettuce; that is, aquatic 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
aquatic 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 hydrophyte, hydrophytic plant, water plant.

water lettuce (noun)	aquatic 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