Disclaimer: This is not safe. This is not well tested. This belongs here, not on the other question. This is weak evidence in collaboration for the drinking seawater claim.
On request by Seth Robertson:
Dr. Bombard made the attempt to sail across the Atlantic in a 15 foot boat called L'Hérétique to prove his claim that a castaway in the Atlantic could survive indefinitely at sea starting with no food or water. Particular quote:
"Now he had been at sea for 9 days and subsisting on up to 1½ pints of seawater with the liquid squeezed from the fish he caught each day. The seawater he said would satisfy thirst provided a person did not wait until dehydrated before starting to drink it. He must start drinking seawater from the moment he is cast away."
The doctor clearly thought he had an explanation for body chemistry as to how the body could extract and dump the excess salt from seawater, among all his dietary analysis that he did beforehand. Unfortunately this source doesn't copy it for us.
If I read the encounter right, there was no rain until 13 days in, but several points describe humidity in the air. He goes to great lengths to ensure there is no seawater on the topside to evaporate and leave solid salt.
Source: Chichester, Sir Frances, Along the Clipper Way, 1966 Chapter 3, pp. 19-34 SBN 345-02103-7-125 (SBN isn't a typo. That's what it says.)
There's some trouble with the attempt resulting in this being decidedly not a controlled experiment. On one hand, we would expect the fish he ate would be providing all the water he needed. On the other hand, he lost 50 pounds on the entire crossing. By the ordinary way of estimating if he's losing weight he's not getting enough water to live on in his food, raw fish or not. But 1½ pints of fresh water wouldn't make up the difference. He had rain starting on day 13 that provided plenty of fresh water from that point. This throws the weight number kinda off.
From time to time I hear another impossible sounding survival-at-sea story. For some reason a disproportionate number of these include the same idea of slowly drinking seawater. I can't imagine how it's really supposed to work. If the raw cooling is worth the salt, than bathing in it would work better.
I have cross-confirmed that Bombard's claim exists and a contemporary peer review resulted in a counterclaim. I can't read it though.
Somebody else's summary provides a rather implausible explanation for how the salt extraction works:
"What does for many castaways is that once adrift and with all fresh water exhausted, it’s only in a state of acute desperation that they turn to sea water (or urine). By now severely dehydrated, the kidneys can’t handle the sudden accumulation of toxins and an agonising death soon follows, supporting the mariner’s lore that drinking seawater was fatal."
Except we know that's wrong. The kidneys can't extract salt into urine more concentrated than the blood. But Dr. Bombard couldn't have known that. It's barely possible that there's something else going on here that really permits this to work, quite possibly only for certain people. But if so, nobody's got a clue.