If you are naked somewhere where you are not supposed to be naked, you could be charged with a lot of different crimes depending on who saw you. For example, indecent exposure is one crime that you could be charged with if an adult saw you naked. If a minor saw you naked, then you could be charged with a lot more serious offenses. Better to keep your clothes on.
Do an internet search on nudist colonies and check out the various websites and their information. There may also be some news stories on local spots where law enforcement is not too concerned with nudity like at Hippy Hollow outside of Austin, Texas.
Public nudity is not prohibited nationwide, it falls to state and local laws to prohibit it. Most states have state laws against public nudity, but some don't. My understanding is California, for example, does not have a state prohibition against public nudity, but nearly all of the cities and counties do. I believe the “nude beaches” in California are actually state beaches that don't fall under any city or county anti-nudity laws. Even states that do not prohibit public nudity entirely usually have laws against offensive sexual displays in public however. And in states that do have anti-public nudity laws, the laws usually actually require someone be offended or the nudity was for the purpose of sexual gratification.
Notice: The information contained herein is intended as general legal information and does not create an attorney-client relationship and is not governed by confidentiality rules. This general legal information is not a substitute for seeking the direct advice of legal counsel.