The sea is the one place where you can be surrounded by water and die of thirst. Every shipwreck story knows that, and it seems plainly unfair. It's water. There's a lot of it. Why not just put up with the taste?
The answer is a piece of bookkeeping, and your kidneys are the accountants.
The sums
Seawater is about 3.5% salt by weight, according to the US National Oceanic and Atmospheric Administration (NOAA). A 250 ml glass, at seawater's density of about 1.025, weighs roughly 256 grams, which works out at around nine grams of salt.
The World Health Organization recommends less than five grams of salt a day for an adult. So that single glass delivers nearly double a full day's limit, and you're only drinking it because you're already in trouble.
The kidney problem
Your kidneys get rid of excess salt in urine, but they have a limit on how concentrated they can make it. As NOAA explains, human urine can only be less salty than seawater. So in order to flush out the salt from a glass of seawater, you have to pass more water than you drank.
Think of a bath. Picture the tap running and the plug out, with the plug draining faster than the tap fills. Each glass raises the amount of water going through while lowering what's left. The level drops, however much you pour in.
The water that makes up the difference comes out of your body's own tissues, as the US Geological Survey puts it. So you become thirstier with each sip, and that thirst is a correct signal that's now telling you to make matters worse.
What it does to your cells
As the blood gets saltier, water moves out of your cells towards it. The cells shrink. The MSD Manual, a standard medical reference, covers this under hypernatraemia: the brain's cells are affected, thirst is the first symptom, and it progresses to confusion, seizures and coma.
A grape turning into a raisin is a good image of a cell shrinking as it loses water. It's only a picture, since raisins are made by drying, not by salt. But it gets the shape of the thing across.
The maths we checked
It's easy to get these sums slightly wrong, so here they are in full:
- 250 ml × 1.025 g/ml ≈ 256 g of seawater
- 256 g × 0.035 ≈ 9 g of salt
- 9 g ÷ 5 g ≈ 1.8 times the daily limit
The exact saltiness varies from one sea to another, and a glass isn't a precise measure, so "about nine grams" and "almost double" are the honest ways to say it.
One thing worth adding
"You can't drink seawater" isn't the same as "you can't ever get drinking water from the sea". Salt can be removed, and that's what desalination and solar stills do, but ordinary household filters aren't designed for it. So if you're ever choosing kit for a boat, check specifically that it says it desalinates.
