Itching red papules that may appear on the skin within a few hours of swimming in saltwater. The rash is caused by the sting of the larval forms of the thimble jellyfish or the sea anemone. The rash is usually more prominent under swimsuits than on exposed skin because the pressure of clothing on the skin releases the stinging barbs of the larvae. The swimsuit should be washed before it is worn again. Treatment is symptomatic, with oral antihistamines or topical corticosteroids.