Only been to 3 beachfront campgrounds, all very good in their own way:
Assateague State Park (MD) - just across the dunes from the ocean, plenty of beach for everyone. No shade in the campground. Only 1 loop has electric. The adjacent National Park is good, too, but no hookups or hot showers.
Huntington Beach State Park (SC) - a short walk via path & boardwalk to the beach, but being back a bit does give most campsites some shade from the trees.
Jetty Park (FL) - you have to cut through the day use parking lot to get to the beach, and the beach is considerably more crowded than the other two parks mentioned above, but it's a pretty good county park for the price, considering how popular Florida is in winter.
I guess there is one more that I stayed at - Kathryn Abbey Hannah Park (Jacksonville, FL) - but didn't really sample the beach since it was early March and rather cool when I passed through. Big park, and you probably need to drive or bicycle to the beach from the campground. Should be good if you're there when the weather is right.
EDIT : Almost forgot Fort DeSoto (FL), although the beach is also a drive from the campground. Great park.
