All the beaches in CA are fair game. About the *possible* only exception would be if the particular beach you are thinking about, is federally administered. Fortunately very few beach in CA are run by fed. parks. And I say *possible*, because, EVEN THEN you'd be hard-pressed to get even so much as a scram.
Most of CA's coastline is state run beaches (with a smattering of city and county beaches). And the state run beaches are historically open game. However, that's not to say that if you ran into a purist state-payroll archie that he might not gripe. Ie.: "heaven forbid" you found a 1958 wheatie They could try to say it runs afoul of state park cultural heritage wording. However, this has been a non-issue. And best kept-that-way. I've heard of a few incidents on state beaches, but they are forgotten and nothing ever becomes of them. So the "realistic" answer is: state beaches are fair game.