Zostera marina vs water plant

Zostera marina and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
Zostera marina	Yes	No	No	No
water plant	Yes	No	No	No

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

Zostera marina: submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts
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 Zostera marina include aquatic plant, hydrophyte, hydrophytic plant.

Zostera marina (noun)	water 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