This is very much a Plantain when it is flowering but before that it can look similar to Sea Arrowgrass or even Sea Pink. Its leaves are narrow and succulent as are the leaves of these other two species.
Once flowering, which is June July and August, it has a flower spike that looks quite similar to Narrow-leaved or Ribwort Plantain and very similar to Buckshorn Plantain although the flower spike is usually somewhat shorter in Buckshorn Plantain. It has lots of yellow conspicuous stamens.
Sea Plantain grows on the established areas of Salt marshes, the region generally referred to as the General Salt Marsh, which is perhaps the most diverse region of the salt marsh and a place where both Sea Arrow grass and Sea Pink are also found so early in the year care has to be taken with identification. It will also grow in sandy areas and from between rocks higher up the beach. The one photographed at the top of the page was growing beside a Sea Loch in Scotland. It was Loch Long, just north of Glasgow.
This is another species which is edible along with Sea Aster, Annual Sea Blite and Samphire, although the only one I have tried is Samphire. Also the seeds are edible.
Coastal Wild flowers 