I once heard that the human body is about the same percentage of salt as the sea, so why shouldn’t people drink seawater?
Actually, the concentration of salt in the human body is about a quarter of what it is in sea water. Drinking a lot of seawater will raise the concentration of salt in your blood and can make you more dehydrated than drinking nothing at all. Let’s say you go down to the beach, fill up a big glass with sea water, and drink it down. The salty water goes into your stomach. Then it enters your bloodstream – and increases the salt concentration in your blood. The job of your kidneys is to filter your blood – to get rid of toxins and wastes, including too much salt – and to expel it through your urine. But human kidneys have to excrete a certain amount of water when they expel wastes.
That’s why you can end up more dehydrated if you drink seawater. Your kidneys have to get rid of that extra load of salt – and that takes extra water. So if you’re stranded at sea it’s better to hope for rain than to drink seawater. Or catch and eat a fish – that’ll give you a less salty source of fluids.