When you mess up your dive, there's always the risk of slapping your skin.
From the denverpost.com
Scuba certification requires making one skin dive to retrieve a rock 10 feet below the surface.
From the DesMoinesRegister.com
Consisting of 70 teachers, students, and other interested parties, the members of the expedition planned to skin dive along the coast of Spain and Morocco, seeking evidence of the lost island.