Updated Aug 14
It isn’t the “only choice” to drink sea water. The other choice is don’t drink it.
The problem is, the delusion that, any liquid will re-hydrate you.
The reality is a principle called “osmosis”.
Try drinking some saltwater. Really, I mean go ahead and do that. I suggest 2 heaping tablespoons of table-salt, in 2 litres of water. You will want more than 2% w/v of saline in the water. Spike it with a little orange juice to try to keep from vomiting. Try to get it down within an hour or two.
Then, you will RUN to the toilet, and rapidly dump the contents of your guts, with every meal that you had in the last several days.
You will also be dumping large amounts of water. Some people do this as a purgative/laxative process.
If your only supply was seawater, and you consumed it for several days, this can kill you.
Osmosis starts with a membrane, that allows some things through (like water) but not other things (like NaCl table salt or sea salt).
The salty water (with high concentrations/amounts of NaCl salt in it) will actually suck water out of the cells in your gut. Salty water sucks fresh water out of your body.
Some sodium may be absorbed by your gut, and your kidneys will also be working overtime, trying to get rid of the excess.
Another fun fact is that, poor hydration can happen with high-sugar liquids. Again, it is about the high amount of solutes (stuff floating around in the liquid) compared to solvent (the water). I have encountered rest-home staff who didn’t comprehend this, and didn’t care, as they shoved high-sugar juice at elderly people with weak kidneys.
Also, drinking your own urine in the desert sounds badass, but is actually a terrible idea, due to the sodium and urea in it.