Synonyms for Eelgrass (same or very similar meaning)
WordNet sense 1 (submerged marine plant with very long narrow leaves found in abundance along North Atlantic coasts):
sea wrack, grass wrack, Zostera marinaWordNet sense 2 (submerged aquatic plant with ribbonlike leaves; Old World and Australia):
wild celery, tape grass, Vallisneria spiralisFrom the ODE community, based on WordNetadd/edit