It can be an act of physiological and psychological desperation. There's thousands of years of recorded history of people dying of thirst drinking seawater as a coping mechanism before the end.

I read somewhere that you can survive on salty water as long as you don't leave it until you are severely dehydrated. You need to let your body adapt.

It was from research on how to survive lost at sea.

Not drinking water at all is a definite death sentence.

Edit: Looking a bit more into this, sea water will also kill you as it does dehydrate because your kidneys will try to deposit excess salt into your urin, but is unable to create urin that is saltier than sea water. You simply start peeing more than you drink.

The question is which option kills you faster?

it fucks your liver