I would think it would be because of the salt in it. Also I've heard of stuff being dumped in the ocean. I've seen a medical waste bag washed up on the Texas Gulf Coast before. That's really disgusting to me.
The salt in salt water will do two things to you. First, it will make you even thirstier. Ever eat a bunch of salted peanuts or pretzels?
But more importantly, it will kill you. The high content of salt in seawater will shut your kidneys down, deplete water from the cells in your body and will result in seizures and ultimately a heart attack.
For a very short period of time, you CAN drink up to a pint of sea water a day.
You couldn't drink the water from other 'Salt' water sources, like the Great Salt Lake, Saltin Sea, Dead Sea, ect. because the salt content is MUCH greater than Sea Water.
Even a pint would do you SERIOUS harm!
Salt water will dehydrate you.
It takes more body fluid (water) to flush the excess salts and minerals from your body than you get in moisture.
Like a cup of coffee or strong sodas, it will take up to 3X the amount consumed to flush the caffeine, salts, ect. from your body.
(ever wonder why ONE cup of coffee or one Pepsi will make you go pee a half dozen times?)
Also, there are often mineral, biological critters, and other stuff that will eat away at your digestive tract and poison your liver and kidneys.
IF, you body can't flush out the excess salt, then you are looking at organ shut down...
Like we have talked about in other threads, there is an easy way to make Sea water, water from urine, plants, even poisonous plants potable, and it's called a 'Solar Still'....
Slow, but VERY effective, and the production of 'Fresh' water requires no further treatment...
You can also BOIL water to distill it, but that requires fuel for the fire, closed capture system for water vapor under pressure, and condensation mechanisms you may not have the hardware or know how for...