Good luck scanning the waters for that one before you leap in.
"How toxic they are is just phenomenally frightening and equally fascinating," says Gershwin.
"Just the lightest brush -- you don't even feel it -- and then, whammo, you're in more pain than you ever could have imagined, and you are struggling to breathe and you can't move your limbs and you can't stop vomiting and your blood pressure just keeps going up and up.
"It is really surprising how many places they occur around the world -- places you would never expect: Hawaii, Caribbean, Florida, Wales, New Caledonia, Thailand, Malaysia, Philippines, India ... as well as Australia."
Is any place safe?
"More and more, places around the world that are suffering from jellyfish problems with tourists are developing prediction systems so that tourists can know when it is safe," says Gershwin.
The irony, she says, is that tourists who avoid a area because of the known risk, may alter their plans to hit a "safe" beach whose officials are merely less up front about the jellyfish situation, putting themselves more at risk.
A common misconception is that places such as Indonesia, Thailand, Malaysia and the Philippines are free of dangerous jellyfish.
"Jellyfish occur in all marine waters from pole to pole and at all depths," says Gershwin. "The life threatening ones are found from about 40 degrees north to 40 degrees south latitude.
"Australia is upfront about its jellyfish dangers, and also assertive in safety management, whereas other places have them, but may understand less about them or, in some cases, just don't want to say. I think tourists need to be very aware of local hazards and not expect to necessarily be provided with information."
What to do when you're stung
Thanks to that infamous "Friends" episode that aired in 1997, millions of people think it's smart to treat a jellyfish sting with urine.
In reality, peeing on a jellyfish sting isn't a good idea.
A report in the Scientific American says urine can actually aggravate the jellyfish's stingers into releasing more venom.
Meanwhile, there's debate over what actually works on a jellyfish sting.
Many doctors say it all depends on whether the sting occurs place in tropical or nontropical waters.
If stung in tropical waters, one should rinse the area with vinegar to deactivate any nematocysts -- the parts of the stinger -- that are still hanging on.
"A freshwater rinse will have the opposite effect," says the Scientific American report. "Any change to the balance of solutes, such as the concentration of salts inside and outside of the cnidocyte [a venomous cell], sets off stinging."
In North America, doctors recommend using hot water and topical pain killers on a sting.
Can we turn this around?
Gershwin says the explosion in jellyfish populations is a visible indicator that life in the oceans is out of balance.
"If we somehow managed to eradicate all jellyfish, then something else -- some other weedy thing -- would find a perfect situation," she says. "So the reason we should care is because they act as a flashing red light."
She says there's no one single factor to blame for the rise in jellyfish populations, but rather places blame on a combination of overfishing, warming water, low oxygen and pollution.
By fishing out jellyfish predators and competitors, humans are creating perfect conditions for jellyfish to multiply.
"Sadly, I am one of a growing chorus of people who believe that, yes, it is too late to turn things around," says Gershwin. "So many species are in such low numbers, and habitats are so badly damaged, that restoring them to their original splendor is simply no longer possible.