Is skinny-dipping illegal

In many places indecent exposure is a crime but there are several legal nude beaches (not recommended) where skinny dipping is perfectly acceptable.

Swimming naked in a private or out-of-the-way lake or pond is OK. Just remember that depending where you are, public nudity can be illegal. Also remember that if you are skinny-dipping with a girl, your male parts are going to shrink right-up in cold water. So be prepared that your total exposure could be pretty tiny.