From DermNet:

Sea bather's eruption is a rash that affects areas of the skin covered by a bathing suit, rather than exposed areas, after swimming in the sea. It is caused by stings from the stinging cells (also called nematocysts) of the larval forms of certain sea anemones and thimble jellyfishes. Sea thimbles are small tropical jellyfish that, even as adults, get no larger than about a centimetre in size. Most cases of sea bather's eruption occur during the summer as it seems to be dependant on water temperature. It affects swimmers, snorkelers, or divers soon after getting out of the water.

Sea bather's eruption has been called sea lice by some. Sea lice is actually due to immature larval forms of parasitic flatworms (schistosomes), penetrating the skin.

So, The way to prevent stings... Go skinny dipping! <img src="http://www.traveltalkonline.com/forums/images/graemlins/Wink.gif" alt="" />