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This is false. Sea water is >3% salt and human kidneys can't produce urine with that much salinity or greater. Since they need more water to extract that much salt, the net effect is dehydration. This rower probably had a desalination machine, or just a big reservoir of fresh water.

(Though there might be some obscure edge case, and if you're about to die of dehydration that a little bit of seawater will buy you a minuscule amount of more time? doubt it)

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If you're about to die of dehydration - and have been rehydrating with fresh water - you'll be low on salt too so a bit of seawater should in principle be in the right direction for both and help. In practice I wonder if your digestive tract might object to the salt water too strongly for this to work though.

If you're about to die of dehydration - from sweating - and have not been rehydrating you're already also hypernatremic (too salty) as well so I sort of doubt it would help. Sweat is less salty than your blood so it increases salt concentration.

It depends. Hypernatremia refers specifically to sodium but doesn’t speak to potassium or calcium. You need all three in proper balance for your body. So you can sweat a bunch and still need to take in more salts because the balance is off.

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