grass wrack vs aquatic plant

grass wrack and aquatic plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
grass wrack	Yes	No	No	No
aquatic plant	Yes	No	No	No

As nouns, aquatic plant is a hypernym of grass wrack; that is, aquatic plant is a word with a broader meaning than grass wrack:

grass wrack: submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts
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 grass wrack include hydrophyte, hydrophytic plant, water plant.

grass wrack (noun)	aquatic plant (noun)
submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts	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