Here are beach camping sites in the San Diego area. If you want to try camping without reservations, I'd recommend calling to check the situation out.
San Clemente State Beach
Address: 3030 Avenida Del Presidente, San Clemente, CA 92672
Telephone: (949) 492-3156
Amenities: showers, telephone, restroom, nature trails, 1 mile of beach
Activities: Surfing, skin diving for lobsters, hiking, body surfing, swimming, fishing
San Elijo State Beach
Address: 1600 S. Highway 101, Cardiff by the Sea, CA 92007
Telephone: (760) 753-5091
Amenities: Showers, restrooms, beach access, table, grill magnificent ocean view
Activities: Swimming, surfing, snorkeling, diving.
San Onofre State Beach
Address: 3030 AVENIDA DEL PRESIDENTE, San Clemente, CA 92672
Amenities: 3 miles of sandy beach, ranger station, convenience store, ranger station, showers, restrooms, fire rings, telephone.
Activities: Hiking, swimming, surfing, fishing
Directions: Take 5 north. Just south of San Clemente take the Basilone exit. Follow Basilone Rd. until it ends at the campground entrance and enjoy fun San Diego camping.
South Carlsbad State Beach
Address: 7201 Carlsbad Blvd., Carlsbad, CA 92008
Telephone: (760) 438-3143
Online Reservations: Camping
Amenities: South Carlsbad State Beach is a large bluff-top campground overlooking the Pacific Ocean with stairs down to the beach, table, grill, restrooms, showers, laundry, store.
Activities: Swimming, surfing, skin diving, fishing