Estuarine pondweed, tassel pondweed or ditch grass (Ruppia maritima)
Contrary to its name it is not a water plant but the pondweed tolerates saltwater. It has a grass-like appearance with long thin (1mm) stems of up to 1 m long and leaves and short flower stalks.
Pondweed is often found in coastal wetlands and estuaries where it grows underwater to form dense clumps that can damage boat propellers. It is an important food for waterfowl.
Most often this plant pollinates itself, but can also be pollinated by floating pollen.
Fish and waterfowl can spread the seeds and fruit but the plant can also grow from its rootstock to form colonies.
Ruppia cirrhosa: which has a coiled flower stalk.
Potamogeton pectinatus: which display its flowers clustered on a spike.