duckweed vs water plant

duckweed and water plant both are nouns.

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

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

duckweed: any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds
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 duckweed include aquatic plant, hydrophyte, hydrophytic plant.

duckweed (noun)	water plant (noun)
any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds	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