Sea grass

Sea grasses are flowering plants which live in the sea. They come from four plant families in the order Alismatales. They are monocotyledons which grow in marine, fully saline environments.
Sea grass (seagrass) is a key part of continental shelf ecosystems where phytoplankton produce carbonate sediment. This kind of ecosystem occurs in subtropical places like Florida Bay, and around Bermuda. Sea grass beds often contain many species from various phyla. Apparently, seagrass herbivory is a highly important link in the food chain. Many species feed on sea grasses, such as green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs.
The surface of the seagrass is the place where "Melobesia", a small alga, makes chalk as a by-product of its metabolism. That is not the only chalk-producing organism in the seagrass beds. 
Largest plant.
A huge bed of seagrass has been found off the coast of Western Australia. 
Researchers collected shoots from the bay and looked at 18,000 genetic markers. It is "all from the same plant", and covers about 77 square miles (200 square kilometres).