On the beaches of New Zealand, swimmers have lately been troubled by hordes of larval thimble jellyfish. I wrote about these troublesome critters in The Book of Deadly Animals:
The larvae of this jellyfish are popularly called sea lice, but they are not related to the tiny crustaceans of that name. Though microscopic, thimble larvae cause a rash which sometimes endures for weeks and brings on secondary infections. The larvae generally sting only when trapped by clothing, a habit which concentrates the stings in the areas of the body covered by the swimsuit. (I pause a moment to let you take in the implications.) The larvae can survive dried out for months. Some people have found themselves attacked all over again when they put on the swimming gear they peeled off and hung to dry the summer before.
As the article linked below mentions, some beaches have been troubled by more formidable cnidarians (as jellyfish and their relatives are called). The portuguese man-o'-war (pictured above) is one such relative, a colony that behaves like a single animal. Its stings are, on rare occasions, fatal.
Jellyfish invade Auckland beaches - National - NZ Herald News:
"A similar outbreak of sea bather's eruption occurred last February as La Nina's warm currents encouraged the spread of the jellyfish on eastern Auckland beaches.
The larvae are usually found in warm, still water and are rarely a problem at beaches with heavy surf, such as Piha.
Dr Baker said the only sure-fire way to avoid the rash was to not swim at affected beaches. But swimmers could lessen their risk by not wearing large, baggy clothing and by removing their togs on leaving the water.
Last month, thousands of jellyfish washed up on Wellington's south coast beaches, including the dangerous bluebottle or Pacific man o' war, sparking warnings from authorities.
And a swarm of bluebottles, including one with 2.5-metre-long tentacles, closed Oreti Beach, near Invercargill, this month."
Giant Jellyfish Attacks New Hampshire
Diana Nyad vs. The Cnidarians