Fort DeSoto park in St Petersburg, FL has plenty of waterfront sites, though not beachfront in the sandy sense. There is a beautiful sandy beach elsewhere in the (very nice) park.
There are many beachfront/waterfront campgrounds around if a lake beach is acceptable, both public and private parks. Still more have riverfront sites. New York, for example, has several state parks on the St Lawrence and several more on Lake Ontario, some of which have sites that are literally at the water's edge. Plenty of others in other states are around the great lakes.