A few months ago I tried the hot new fad of sensory-deprivation float tanks. I did not achieve any kind of Buddhist nirvana or mind-expanding hallucinatory state, but I did get to lie in a darkened coffin-like thing for an hour in body-temperature water. It was a little boring; next time I might do paintball instead.
To keep you floating right at the surface of the water, these places dissolve a ton of "Epsom salt," magnesium sulfate, in their water. Denser water means more buoyancy. You've probably seen the same effect when you've been swimming at the beach: it's much easier to stay afloat in salt water than fresh water.
The saltiest body of water in the world (if we ignore the hypersaline ponds of Antarctica's Dry Valleys) is famously the Dead Sea on the Israeli-Jordanian border. Evaporation in the hot, dry desert there at the world's lowest point makes for water ten times saltier than the ocean, and tourists love to come and bob like a cork at its beaches. According to popular wisdom, it's impossible to drown in water that salty.
But by the numbers, the Dead Sea is one of the most dangerous places to swim in Israel. Lifeguards pull over a dozen stricken swimmers from its waters every year, and at least four people have drowned there since 2016, including a wealthy oligarch/politician from the Russian far right. Begin your conspiracy theorizing now!
How do people get in trouble in water that salty? Well, precisely because the water's so salty. Floating on your back, you're fine, but if you get turned over, it's hard to get an arm or leg down through the dense water and right yourself. If you panic at that point and swallow some of the super-salty water, you can do immediate damage to your heart and kidneys. In fact, dehydration and electrolyte imbalance can set in so quickly in hypersaline water that experts recommend not staying in the water for more than twenty minutes. You don't want the name "Dead Sea" to become literal.
Quick Quiz: The Dead Sea is the "terminal lake" for what famous river, which flows into its north end?