To identify national park mystery spot 16, you were told that you could:
Shuck all your clothes and wade where it's shallow
On a shoreline named like a chasm in shadow.
Rangers will not bother you there,
And other bathers will not stop to stare
The mystery spot is Gunnison Beach in the Sandy Hook (New Jersey) unit of Gateway National Recreation Area. Gunnison is a clothing-optional beach that attracts up to 5,000 nude sunbathers and swimmers on some summer days. The area open to naturists is clearly marked with signs. The beach itself is lifeguarded and has abundant concession stands. A local naturist group raised funds for a boardwalk that provides better access to the beach.
The Black Canyon of the Gunnison, the namesake landform of Colorado's Black Canyon of the Gunnison National Park, is the "chasm in shadow" referenced in the rhyme. This great canyon is so deep and narrow that its walls are heavily shaded and dark-looking most of the time.