I wish I could remember the source of the quote, but I once read that the difference between nude and naked in intent on the part of both the artist and the viewer.  Nude is simply without clothing, not trying to elicit any type of salacious response.  Naked works in the other direction.

Think "Venus Rising from the Waves" vs Penthouse magazine.  There is a purity to the one image that is not even implied, and often mocked, in the other.