The short answer would be anytime the water is nice, considering algae blooms, wind, swell, etc.
diving in California is year around usually with fall being best and spring being windy. But I've seen wonderful days in winter and crappy days in fall so sometimes there is no rhyme or reason.
Tropical storms from Mexico can have an affect in SoCal waters causing some big swells.
just book a trip and go. If it gets cancelled for any reason they will refund your money.
Just for fun shallowish dives I like to go on the Spectre out of Ventura, they go to Anacapa Isl.
It's a little north from LA. $130 gets you 3 dives, free air, and lunch and snacks. They also let you sleep on the boat the night before so no motel fees. Free parking too and great restaurants and shops on the wharf.