If you are thinking of enjoying one of California's clothing-optional beaches, you need to know about the laws before you go. It's a complicated situation, and the law depends on where you go.
You can avoid legal hassles by going instead to a privately-owned clothing optional resort in California.
Nudity on Federal Land
There is no Federal law against nudity, but neither is it a guaranteed right. This means that state, county, and local laws can take precedence.
Federal land includes Point Reyes National Seashore and beaches around San Francisco which are in the Golden Gate National Recreation Area.
California State Nudity Law
This law applies to public places anywhere in California. A 1972 California court case ruled that mere beach nudity is not indecent exposure. However, different localities have strict anti-nudity laws (notably Santa Barbara and Los Angeles Counties). In those places, beach nudity will net you a citation and a fine with no questions asked.
For other counties where there are clothing-optional beaches, the laws are summarized in the county beach guides, which you can access through this index.
CALIFORNIA CODES SECTION 314-318.6 (as of 6/2016) says:
"Every person who willfully and lewdly, either: 1. Exposes his person, or the private parts thereof, in any public place, or in any place where there are present other persons to be offended or annoyed thereby; or, 2. Procures, counsels, or assists any person so to expose himself or take part in any model artist exhibition, or to make any other exhibition of himself to public view, or the view of any number of persons, such as is offensive to decency, or is adapted to excite to or thoughts or acts, is guilty of a misdemeanor."
It goes on to say:
"Every person who violates subdivision 1 of this section after having entered, without consent, an inhabited dwelling house, or trailer coach as defined in Section 635 of the Vehicle , or the inhabited portion of any other building, is punishable by imprisonment in the state prison, or in the county jail not exceeding one year.
Upon the second and each subsequent conviction under subdivision 1 of this section, or upon a first conviction under subdivision 1 of this section after a previous conviction under Section 288, every person so convicted is guilty of a felony, and is punishable by imprisonment in state prison."
Nudity Rules in California State Parks
Many of California's beaches are in state parks. Enforcement of these policies varies by area and is more likely where clothed visitors complain to the park rangers.
Section 4322 of Title 14 of the California Administrative Code regarding nudity in the state parks says: "No person shall appear nude while in any unit except in authorized areas set aside for that purpose by the Department. The word nude as used herein means unclothed or in such a state of undress as to expose any part of portion of the pubic or anal region or genitalia of any person or any portion of the breast at or below the areola thereof of any female person.
All sections are misdemeanors which carry a maximum punishment of 90 days in jail and/or $1,000 fine."
The regulation allows for setting aside specific clothing-optional areas, but the parks department hasn't taken advantage of that. Instead, State Park Rangers have operated for many years under a policy known as the "Cahill" policy, named after a former Parks Director:
"it shall be the policy of the Department that enforcement of nude sunbathing regulations within the State Park System shall be made only upon the complaint of a private citizen. Citations or arrests shall be made only after attempts are made to elicit voluntary compliance with the regulations."
However, when the Cahill Policy was written, the public stance on nudity was more liberal than it is now. You can find more details about it at the Bay Area Naturists website.
The people most likely to make a complaint are also the least likely to make an effort to get to a remote part of a state beach, so those areas have become clothing optional. If you stay in those areas, you are unlikely to be bothered. If a Ranger appears and asks you to put your clothes on, don't argue. Comply and remain clothed for the rest of the day to avoid a citation. If you cross the line from mere sunbathing to other activities, you can (and probably will) be prosecuted under California Penal Code Section 314, which is quoted above. Anyone convicted will carry a lifetime registration as a sex offender.