A French biologist Alain Bombard as proved with a 65 day trip across the Atlantic that one can drink sea water up to one liter per day if one can drink raining water and water extracted from fishes too. So, yes, you can drink sea water, but not too much. So, do not keep in mind that you absolutely cannot, as it can bring you an extra day of survival (if you do not have other source of water).
I down voted because you give a dangerous advice. The concentration of salt in sea water is higher than in urine. Drinking sea water rises the level of sodium in blood very quickly which sends signal to your kidneys to get rid of that salt. But as kidneys can't produce urine with the concentration of salt higher than certain level you need more body water to excrete the amount of salt you consumed with sea water. Roughly speaking you need 1.5 liters of fresh water to excrete the salt you got with 1 liter of sea water. Eventually the concentration of salt will become dangerously high in you blood and you will die of heart failure or seizure. So I would say that drinking sea water will actually DEPRIVE you of extra day of survival which may be lifesaving.
Modern survival guides consistently advice against drinking sea water.
Speaking of Alain Bombard. The most likely explanation here is that he consumed enough FRESH water (with fish and rain water) to dilute sea water he drank to safe levels for his body.
So if you are on a boat in the middle of the sea NEVER drink sea water. This can save your life.
Actually if you're dying of dehydration, chances are you're somewhere exceptionally hot, because it's rare that you wouldn't be able to find water in 3/4 days in a temperate climate.
If you're in an area where you're perspiring a lot, then occasionally drinking sea water would actually be beneficial as there aren't many readily available sources of salt.
I work construction, in the longest heatwave of the summer I was putting tablespoons of salt into my water jug as it's quite easy to become nauseous when drinking a gallon of water inside a work day. Not to mention you don't want to get disorientated or light headed whilst 30ft in the air.
Long time survival in a hot environment requires a source of both fresh water and salt. The ocean is a far easier source of salt than searching for anything in nature.
Well the problem is just as the OP says: human cells have a low salinity. So pure sea water is bad, since it has a much higher salinity; but mixing it with fresh water (as that French guy mentioned by the parent apparently did) is ideal, as it brings the salinity level closer to the mark.
It's difficult for me to drink fresh water in large quantities (more than a liter an hour) but when I put electrolytes in them, I find I can drink somewhere on the order of 2x to 3x more.
I buy these 100 at a time - seems to do the job pretty well.
http://en.wikipedia.org/wiki/Alain_Bombard