Why the kidneys cannot make seawater “count” as water
After you drink seawater, its salt is absorbed into your bloodstream. The kidneys then have to remove the extra sodium and chloride to keep the blood’s chemistry within a workable range. But they cannot pack salt into urine without water: urine must carry dissolved waste out of the body.
Because seawater is more concentrated than the urine human kidneys can produce, flushing out its salt costs more water than the drink supplied. The water deficit comes from the body’s existing supply, including fluid in the blood and tissues.
The body’s response has limits
As the blood becomes more concentrated, sensors in the brain prompt thirst and the release of a hormone that tells the kidneys to conserve water. That response reduces water loss, but it cannot make the kidneys excrete seawater’s salt without spending water. Meanwhile, the salt still has to be cleared.
So seawater can briefly add fluid to the digestive system while leaving the body with less usable water overall. The memorable catch is that the ocean’s salt does not merely fail to hydrate you: removing it forces your kidneys to draw on the water you already have.
