So Cal resident here. I've found that most places off the coast is a great spot. I'd look for Kelp patties on the water while driving along the coast. Populated areas are not so great, so avoid big areas like Santa Monica pier. Places like Malibu are great and also palos Verdes. If you've got the cash for it, going to Catalina or Channel Islands are great spots as they're less trafficed imo. Biggest point though, if you go SCUBA out here make sure the wave action and wind are lower, since that makes for better visibility! Stay safe out there!