In addition to those already mentioned, Jetty Park in Cape Canaveral, Florida has an excellent beach, although you have to cross the day-use parking lot to get to it from the campground.
Assateague State Park, just south of Ocean City, MD, is great too, although only 1 loop has electric, and that's probably already fully reserved for the summer. Plenty of options if you can get by with a generator. Miles of beach used by only a few campers, very uncrowded. The adjacent National Park has no services, but lots of beach, too.
First Landing State Park in Virginia Beach, VA is just barely inside the mouth of the Chesapeake Bay and has electric & water sites, with plenty of sand & water. Feels like the ocean.