A short dig led me to this entry from Merriam-Webster's Medical Desk Dictionary. Seabather's eruption is:
acute pruritic dermatitis that occurs on parts of the body covered by a bathing suit within 24 hours after exposure to seawater containing certain tiny coelenterate larvae (as of jellyfishes, sea anemones, or corals) and that is caused by nematocysts fired by the larvae caught in the mesh of the bathing suit or compressed between the bathing suit and the skin.
What does that mean, you ask? Let me translate the jargonese for you. Simply visualize this set-up:
You've been swimming in the ocean. Your skin starts to itch and swell, but only under your bathing suit, and only
some time after you've been out of the water. This is not a sudden onset of chicken pox. The symptoms' roots lie in the miniscule baby jellyfish and other tiny life forms with which you've been swimming. Those little demons zap your bathing suit and fill it full of their evil stingers. Ready for more? You can't shower them out of your suit: freshwater triggers the poisonous zappers. (Zappers is a medical term I picked up while learning about this minor health problem.) You can't let your suit dry while you're wearing it either. You know why, don't you? Correct! This will also magically release the zingers. (Zingers are similar to the horrid little jokes I like to crack as I torture my rapt audience with fun yet nuggetty knowledge.)
What's so fabulous is the simple solution. If you've been swimming in waters that are prone to this, such as the Florida Gulf Coast, simply change out of your suit after swimming and rinse yourself off with a solution of ¼ cup white vinegar to 1 1/2 cups water.
I suppose that I'm so accustomed to our tranquil Mississippi Gulf Coast that I never think of the little beasties within. I think I'm glad that beach season is over for the year so that I won't have to swim and think about them anytime soon. Let's make a pact. I'm going to forget all about this, and you won't remind me next year before I go on vacation.
Merriam-Webster's Medical Desk Dictionary, Revised Edition. Springfield: Merriam-Webster, 2005. Retrieved from http://www.credoreference.com/entry/mwmedicaldesk/seabather_s_eruption
Formichelli, Lindo Health Jun2007, Vol. 21 Issue 5, p84-86, 2p