/sci/ - Science & Math

There's more to getting water into your blood than simply drinking it. It has to be absorbed into a body through a process called osmosis. Osmosis is is the transfer of water from a low salt environment to a higher salt environment. Fresh water naturally gets sucked into the blood because the blood is more salty. The kidneys are also able to compensate for salty water to a degree, by using water from the blood to flush out the salt, then reabsorbing the diluted water. The kidneys are able to do this for water that is up to 2% salt.

Unfortunately, sea water is is 3% salt. The kidneys need to spend more water diluting it than they get from absorbing it, so seawater dehydrates the body.