Saltwater is obviously wet. And we eat things with salt in them all the time. So why can’t we drink saltwater?
It isn’t really because of germs (though that’d also be a valid reason). It’s mainly because…well, it’s salty. Sodium is the main element in salt, but salt isn’t inherently bad for you. It’s actually an electrolyte that’s vital in helping your body regulate blood volume & other functions). Whatever salt your body doesn’t need gets filtered out through the kidneys in the excretion process, which requires water. That’s why, after eating a big bag of potato chips, you probably feel super thirsty. Your brain says “drink more water” so that your kidneys can flush out all that extra salt.
While seawater has both salt & water, it’s too salty for your body to process using just the water that’s in it. Seawater’s salt concentration, or salinity, is around 35 grams per liter. Meanwhile, your blood’s salinity level is only about 9 grams per liter. So, seawater is 4 times saltier than your blood. So, if you were to drink saltwater, that huge increase in salt would shift the water that you need inside your cells to your blood, which quickly affects how the brain functions. It can actually cause death.
Essentially, the sodium & water inside your cells have to be balanced with the sodium & water outside your cells. But, when seawater sodium enters your bloodstream, your cells will try to maintain the balance by dumping their water into your bloodstream. Then, your kidneys will use that water to excrete as much sodium as possible, which is what can cause extreme dehydration.
What if you accidentally swallow some seawater during an ocean swim? While that can bring on mild dehydration (including dry mouth, dizziness, fatigue, etc.), you can counteract the effects by drinking fresh water. But, if you were to get stranded at sea with no fresh water in your vicinity, drinking seawater would absolutely be a terrible call, worse than drinking nothing at all. Gulping saltwater would actually cause a much more severe, life-threatening form of dehydration. In that survival situation, rainwater is what you should look for. Or you could also look for fish eyes, which contain low enough salt levels that they could actually help hydrate you, as would the flesh of fish, birds, and turtles. (So, maybe just avoid this situation by keeping your head above water & your feet on solid ground.)
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