I feel like I’ve always ‘known’ that it’s bad to drink seawater but I can’t remember ever learning why. This excerpt from Rose George’s excellent ‘90% of Everything: Inside Shipping‘ gives a fair idea:
In a lecture to the Royal College of Physicians in 1942, MacDonald Critchley, a physician who had studied survival at sea, said that “seawater poisoning must be accounted, after cold, the commonest cause of death in shipwrecked sailors.”
At first, it wouldn’t seem so: seawater is liquid and it quenches. The relief would be immediate. But seawater has an average salt content of 3 percent. This increases thirst dramatically so that more seawater is drunk, and more, and salt levels go ever more haywire, until the body tries to regulate it by urination, and you expel a quart of urine for every quart of seawater drunk, making matters worse. There are also complicated and intricate effects of seawater on cells, blood, and tissue, but in essence, too much seawater can fry your brain.
Then this happens, in the words of Critchley:
“The victim becomes silent and apathetic, with a peculiar fixed and glassy expression in the eyes. The condition of the lips, mouth, and tongue worsens, and a peculiarly offensive odor has been described in the breath. Within an hour or two, delirium sets in, quiet at first but later violent and unrestrained; consciousness is gradually lost; the color of the face changes and froth appears at the corners of the lips. Death may take place quietly: more often it is a noisy termination, and not infrequently the victim goes over the side in his delirium and is lost.”