There is no federal law against nudity. The legality of various forms of undress is currently left up to the individual states and localities to decide. Nudity is generally allowed in national parks, unless local laws have overriding jurisdiction. 

In 1981 (Schad v. Borough of Mount Ephraim, 452 U.S. 61, 65-66), the U.S. Supreme Court ruled on nudity, as it appears in movies, plays and TV. They found that: "Entertainment, as well as political and ideological speech, is protected; motion pictures, programs broadcast by radio and television, and live entertainment, such as musical and dramatic works, fall within the First Amendment guarantee ... Nor may an entertainment program be prohibited solely because it displays the nude human figure. 'Simple nudity' is generally protected by of the First Amendment. This may be interpreted as protecting a naturist's right to engage in a nude demonstration or public performance on a beach. Local law may override this ruling.

California State Law

As long as nudists are in a reasonably secluded area, it is legal unless and until a complaint from a member of the public is received. A person may be guilty of being a public nuisance if, when asked to dress, fails to do so. Upon such complaint a warning is to be issued and, if not heeded, a violation (of Title 14, California code of Regulations Section 4322) has occurred. Further activities of a person so warned are prohibited for the remainder of that day. Activities on later days are proscribed only if preceded by a new public complaint and renewed warning.

A 1972 California Supreme Court decision In re Smith held that simple beach nudity (absent sexual motives) does not constitute the crime of indecent exposure. In re Smith (1972) 7 C3d 362, 102 Cal Rptr 335, 497 P2d 807 (excerpt): 
Nudity is not considered a lewd act within the meaning of Penal Code Section 314, condemning indecent exposure, unless sexual conduct is involved. Accordingly, a conviction of that offense requires proof beyond a reasonable doubt that the person was committing a sexual act. Absent additional conduct intentionally directing attention to his genitals for sexual purposes, a person who simply sunbathes in the nude on an isolated beach does not 'lewdly' expose his private parts within the meaning of Penal Code Section 314, condemning indecent exposure.

CFI's Conclusion:

Cater to others by using reasonably secluded areas. Cover up when asked or ask other others if they mind that you or your family continues to skinnydip or sunbathe nude. Showing respect for others will increase their respect for you and your lifestyle. Remember, some people think nudity equals sex and we all need to help in the public educational process by setting a good example.

Positive naturist values provide a healthy and wholesome atmosphere for the whole family that allows everyone, including children, to have a better body image, learn respect and have more self esteem. 

Sure, some people might believe they are offended by nude bathers, but, if you never encounter anything that offends you in your community, you are not living in a free society.