eelgrass

Short definition from Wiktionary

Noun

1. 1
   Any of several species of aquatic plant, with very long and narrow leaves: Zostera spp. in marine environments
2. 2
   Zostera spp. in marine environments: especially Zostera marina, growing abundantly in shallow bays along the coasts of the Northern Hemisphere.
3. 3
   Any of several species of aquatic plant, with very long and narrow leaves: Vallisneria spp. in freshwater environments.