Ocean itch: An intensely itchy rash due to contact with the tiny thimble jellyfish (Linuche unguiculata).
These jellyfish are common between March and August in the waters off of Florida and in the Gulf of Mexico and the Caribbean Sea. There may be no adult jellyfish around as a warning. The jellyfish larvae look like mere specks of "finely ground pepper" and can evoke the same response.
The reaction tends to start 4-24 hours after exposure to the jellyfish. People who have had previous exposure to seabather's rash may have an immediate stinging sensation. Some people feel like they have the flu with nausea, vomiting , headache, muscle and joint aches, and malaise.
A bathing suit traps the jellyfish larvae with the fabric acting like a net. The best way to prevent stings is clearly to stay out of the water. Anyone who has had a previous episode of seabather's itch is advised to not go in the water. If one goes in the water, one can wear clothes such as a wet suit that provide a protective barrier. Careful washing of swimwear after taking a dip is advisable. Wearing a T-shirt into the water is a poor idea because it increases the risk of a severe reaction. Topical anti-itch creams are only temporarily effective.
Other names for this condition include bather's eruption, sea poisoning, and seabather's itch and the jellyfish are sometimes called sea critters or, incorrectly, sea lice.