For how long can a person drink sea water?
(source: free.fr)  

Alain Bombard

He is a french biologist who voluntarily tested how many days a man can survive drinking seawater and how?

Biologist point of view

Sea contains ~3.5% of salt$^1$. Our kidney separates the waste from water and excrete them in urine provided the salt content is less than ~2%.$^2$
So, it will take the water already present in the body. Causing excessive thirst and dehydration, eventually cause death.

Alain's Experiment


  Just one spoon of seawater at 20 minute intervals, drinking very
  slowly and letting the saliva in your mouth reduce the saline in the
  water you have swallowed.


Though he survived 65 days (From and including: Sunday, 19 October 1952
to Tuesday, 23 December 1952, ~4,400km) of his journey and lost 25 kg of weight, this result is never successfully reproduced. 

Source:


http://oceanservice.noaa.gov/facts/whysalty.html
http://paradise.docastaway.com/drinking-sea-water