What happens to your cells when you drink salt water?

nothing...

the main problem with drinking salt water is that it dehydrates you more as you drink it (instead of hydrating you) due to the large amount of salt inside the water.

thus, drinking salt water will only aggravate your problem if you're stuck on an island with no fresh water.

not to mention that if you're drinking water from anywhere that has boats or...anything pass through it, chances are the water will be filled with chemicals or other nasty things.