There is a place in Maryland to camp on the beach. Assateague Island has a State Park and a National Park.
Assateague State Park has paved campsites on the beach. Each loop has a masonry bath house with showers, toilet facilities and camp sinks. It is dry camping except for loop H which has electricity. Better get on it because they go fast.
The National Park has dry camping with chemical toilets.
There are also sites classified as "Bayfront" they are a short distance from the beach and in more of a grassy area.
Now, some caveats. Bugs can be bad... really bad, and the Bayfront sites are even worse, bring DEET. Our friends decided to set up a screen room one trip. We quickly learned that the wild horses that live on the island know that people keep goodies in those screen rooms and went right through it, in one side and out the other. There are signs warning that wild ponies bite and kick.
Click for pictures and trip report.