Eelgrass also known as Zostera is a small genus of widely distributed seagrasses.  Commonly known as marine eelgrass, seagrass or eelgrass.  Fishermen and recreational boats call it seaweed.  

 Facts about Eelgrass:

• Found in estuaries usually submerged or partially floating 
• Most species are perennial
• Beds are nursery grounds for many species of economically important fish and shellfish.
• Important food for brant geese and wigeons (type of duck), and even (occasionally) caterpillars.. 
• Is widespread throughout seashores of much of the Northern Hemisphere as well as Australia, New Zealand, Southeast Asia and Southern Africa . 
• Eelgrass has been used for food by the Seri tribe of Native Americans on the coast of Sonora, Mexico
• Has also been used as packing material and as stuffing for mattresses and cushions.

• Used for thatching roofs. 
• In the United States, eelgrass insulation was commercially marketed in the early 1900s.  Due to an outbreak of a disease which destroyed crops of eelgrass, combined with the collapse of the home building industry due to the great depression, it went out of production and was replaced in new homes with fiberglass (introduced in the late 1930s).
• Studies show promise for eelgrass meadows to sequester atmospheric carbon to reduce anthropogenic climate change.

More info can be found found here at  The Importance of Eelgrass | NOAA Fisheries