If you told me there's a place where I can swim with jellyfish and not get stung, I'd tell you that you're shadier than a Craigslist posting. But it's true. Jellyfish Lake on the island of Palau is an enclosed body of water that contains hundreds of the suckers, sans stingers. They have no predators in their private lake, which is why they've lost their natural defenses over the course of millennia. Save your urine for another aquatic emergency.
So go ahead: Mosey on down to Palau, slap on some snorkel gear, and jump in Jellyfish Lake. You can make faces at them all you want - they can't get you back. Evolution is cruel like that.