We are on our way to Salisbury State Reservation (MA) so I can't comment on the sites yet but we recently enjoyed:
Scusset Beach State Reservation (MA) E/W near Cape Cod and Boston, short walk to beach and cape cod canal
Horseneck Beach State Reservation (MA) My favorite so far - dry camping but sites right on the beach
Hammonasset State Park (CT) E/W short walk to the beach
Best of Previously enjoyed:
Bahia Honda State Park - Keys, sites on the beach
Long Key State Park - Keys, sites on the beach
Gamble Rogers State Park - FL, sites with view of beach
Fort Pickens Fed Park - FL, short walk to beach
St. Andrews State Park - FL, short walk to beach
St. George Island - FL, short walk to beach
Hunting Island State Park - SC, on the beach my favorite beach park ever
Assateague National Seashore - MD, my favorite fall camping spot
