I’m dying of thirst. Can I start drinking seawater?

May 29, 2012 · Castaway Survival

Before, when I saw a castaway having a bad time, literally dying for a drink but surrounded by seawater, the message in films was always ‘whatever happens, never start drinking seawater. If you were to do so, perhaps you’d feel a bit better, a bit less thirsty, but it wouldn’t be long before you were dead. In films nobody explained the reason for this warning, but neither did anyone doubt it was true.

Let’s explain why.

Imagine that you’re drifting on a piece of wood in the middle of the ocean. Your mouth is dry. You are totally surrounded by seawater. The sea is the most amazing blue… It’s blue because it reflects the colour of the sky. Sometimes the sea seems to be a greenish colour and that’s because of the tiny bits of seaweed which make up the phytoplankton which is also green, as are all plants that perform photosynthesis… But then you can’t stand it anymore, you give in and start drinking sea water.


Seawater makes up 97.3 % of the Earth’s water so only 2.7% is fresh water. You convince yourself that there can’t be so much poison in the world, but there is.

Our kidneys are the filters that separate the waste material in the blood and this waste is then stored in the form of urine in the bladder, ready to be expulsed from the body. A kidney cannot make urine from a concentration of salts of more than 2%. Seawater is made up of approximately 3% salt, so if we drink it to quench our thirst the kidneys have to use existing water from our body in order to dilute the extra salt, which in turn makes us feel even thirstier.

This is due to the osmosis process where the kidneys can only generate urine which is less salty than the water we drink. Consequently, the body tends to urinate more to be able to eliminate the excess of salt but instead of elimination, the result is the formation of more salt as the urine obtained is always less salty than the original. The body would have to expulse more liquid than the amount drunk, which is impossible and that’s why the effect of drinking water from the sea is, paradoxically, dehydration.

Seawater contains three percent salt. This means that if we drink a litre our kidneys would need at least one and a half litres of fresh water to dilute all the salt, so this means that they would be forced to extract an extra half a litre from our body.


This is the interesting part. Should you find yourself at high sea and it’s been a few days since it last rained, it would be possible to drink small doses of seawater, just one spoon of seawater at 20 minute intervals, drinking very slowly and letting the saliva in your mouth reduce the saline in the water you have swallowed. A man named Alain Bombard demonstrated this a few decades ago in an experiment called “Voluntary Castaway”.


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{ 17 comments… read them below or add one }

MaRvic C. BriÑan September 15, 2012 at 1:17 pm

nyc one!


Peter Jhon Campaner September 24, 2012 at 8:42 am



Somebody April 19, 2013 at 12:00 pm

its Fab!!!!!!


Somebody April 19, 2013 at 12:02 pm

its nice! it helped me in my project. ThanQ very very very very very very very very very mmmmmmmmmuuuuuuuucccccccchhhhhhhhh !!!!!!!!!!!!!!!


Alvaro Cerezo April 29, 2013 at 9:44 pm

You are welcome!


Wiki June 13, 2013 at 9:15 pm
Andrew Whittaker June 4, 2014 at 11:53 pm

Something everyone knew. but either way…interesting/…


Amardeep Singh July 11, 2014 at 4:17 pm



Chee Woon July 22, 2014 at 4:45 pm

Good article!!


Raly Todorova August 3, 2014 at 10:15 am

So if you get shipwrecked in the middle of the Black Sea, where the salinity is ~1.7%, drinking the water won't have you dehydrated?


Mohammad Bilawal August 6, 2014 at 3:07 am

We can drink sea water from the bottom of the sea if we are a little further and we can store it in something,


Alvaro Docastaway August 29, 2014 at 8:00 pm

Thanks Brett :)


Alvaro Docastaway August 29, 2014 at 8:00 pm

Thanks Amardeep :)


Alvaro Docastaway August 29, 2014 at 8:00 pm

Thanks Chee :)


Alvaro Docastaway August 29, 2014 at 8:01 pm

Probably yes. Even more dehydrated if you get adrift in the dead sea :)


Alvaro Docastaway August 29, 2014 at 8:03 pm

Sorry Mohammad. I could understand what you said.


Mohammad Bilawal August 29, 2014 at 9:52 pm

Alvaro Docastaway You can drink the sea water if you go above the sea level like a little further from the starting of sea and store it in a bottle from the bottom of the sea as its pure from down side not the upward. I hope it helps bro :)


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