Soon after you learn not to eat sand, you’re taught the next crucial lesson of beach safety: Don’t drink seawater. Not because it harbors germs (though that’s also probably a valid reason to keep your mouth shut while dodging waves), but because it’s salty.
Sodium, the main element in salt, isn’t intrinsically bad for us in. In fact, it’s a vital electrolyte that helps regulate blood volume and other body functions. And thanks to our kidneys, whatever salt we don’t need gets filtered out through our urine—a process that requires water. That’s why, after you eat a massive bag of potato chips, you might feel desperately thirsty: Your brain is telling you to drink more water so your kidneys can flush out all that extra salt.
Seawater contains both salt and water, of course, but it’s too salty for your body to process using that water alone. According to Dr. Christanne Coffey, an assistant clinical professor at University of California San Diego’s department of emergency medicine and director of its wilderness medicine fellowship, seawater’s salt concentration (salinity) is approximately 35 grams per liter. Our blood’s salinity, on the other hand, is just 9 grams per liter.
“That means seawater is four times saltier than blood,” she tells Mental Floss. “If you were to drink saltwater, this huge increase in salt would [shift] the water that we need inside our cells to the blood, which quickly affects our brain functioning and can hasten death.”
Basically, the sodium and water inside your cells need to be balanced with the sodium and water outside your cells. When the sodium from seawater enters your bloodstream, your cells will try to maintain the balance by dumping their water into your bloodstream. Your kidneys will use the water to excrete as much sodium as possible through your urine—resulting in extreme dehydration.
If you accidentally swallow some seawater during a choppy surf session, you can avoid the symptoms of mild dehydration—dry mouth, infrequent urination, dizziness, and fatigue—by downing fresh water, too. But if you’re a shipwreck survivor stranded at sea with no fresh water in sight, drinking seawater would quite literally be worse for you than drinking nothing at all. Guzzling saltwater would cause a much more severe, and life-threatening, form of dehydration. “This will rapidly lead to vomiting, delirium, and hallucinations,” Coffey says. “None of which is welcome in a sea survival situation.”
But maybe a better idea is to just keep your head above water.