Speaking for South Carolina...Edisto Beach State Park has half of the Park on the beach and the other half on the marsh.
It fills up fast and is pretty expensive.
Huntington Beach State Park in Murrells Inlet and Myrtle Beach State Park are in the general area of the beach.
There are also two or three commercial ocean campgrounds near Myrtle Beach; Ocean Lakes, Pirate Land and Lakewood Campgrounds.
Other than Edisto, Huntington would be my pick because there is a “castle” there (google Atalaya).
If you can deal with not being on the ocean but close to it, Buck Hall Recreation Area in McClellanville, SC is about 15-20 minutes from the Isle of Palms and you’re smack dab on the intracoastal waterway. Buck Hall from what I gather is like a hidden gem; few know about it.
Attached are a couple pics of Buck Hall: