It is unclear in many states whether nudity per se is indecent. Vermont, for instance, prohibits only "open and gross lewdness and lascivious behaviour", and simple nudity, for instance to swim or to change clothes, does not rise to that standard. Unless the intent is to shock, offend or arouse, a complaint is unlikely to be sustained there.
In the National Forest surrounding the Cog, Federal rules govern, and the NPS has a regulation forbidding nudity; that's how 'mooning the Cog' defendants were prosecuted. As I've said before, if I were ever to attempt to follow that tradition, I think that I'd applique a large crescent moon upon my baselayer, and drop my trousers while exposing only my contempt.