Best diving near San Clemente is offshore around Santa Catalina Island and San Clemente Island. There are dive shops on Catalina, as well as on the mainland in San Clemente and Oceanside. Here's a link to a good map of dive sites around Santa Catalina: http://www.shellbackdon.com/divemap.pdf. Some good dives near San Clemente are Northwest Harbor, where you'll see the wreck of the USS Butler, plus the shallower depths of the Boilers.