Although it is clear that a human cannot survive on seawater alone, some people claim that one can drink up to two cups a day, mixed with fresh water in a 2:3 ratio, without ill effect. The French physician Alain Bombard claimed to have survived an ocean crossing in a small raft using only seawater and other provisions harvested from the ocean, but the veracity of his findings was challenged. In Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 40/60% ratio. A few years later another adventurer named William Willis claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost his water supply. 
Most modern ocean-going vessels create drinkable (potable) water from seawater using desalination processes such as vacuum distillation, multi-stage flash distillation, or by the use of reverse osmosis. However these processes are energy intensive, and most were not available or practical during the age of sail.
Alain Bombard (October 27, 1924 - July 19, 2005) was a French biologist, physician and politician famous for sailing across the Atlantic Ocean in a small boat.
Alain Bombard was born in Paris. He theorized that a human being could very well survive the trip across the ocean without provisions and decided to test his theory himself in order to save thousands of lives of people lost at sea.
Bombard reports he survived by fishing (and using fish as source of both fresh water and food) with a self-made harpoon and hooks and harvesting the surface plankton with a small net. He also drank a limited amount of seawater every day. On the October 23, 4th day of the journey Bombard had to mend a torn old sail, while the backup sail was blown away. On the 53rd day of the journey he encountered a ship. The crew told him that he was still over thousand kilometers short of his goal. However, after the ship's crew offered him a meal, Bombard decided to go on. Bombard reached Barbados December 23 1952 after 4400 km of travel. Bombard had lost 25 kg and was briefly hospitalized. He published a book entitled "Naufragé Volontaire" about his trip in 1958.
Bombard's claim was later tested and contested by Hannes Lindemann, a German physician, canoeist and sailing pioneer. Lindemann wanted to repeat Bombard's trip in order to gain a better understanding of living on salt water and fish, but found that he needed fresh water (from rain) most days. Lindemann later claimed that Bombard had actually taken along fresh water and consumed it on the ocean, and that he had also been secretly provided further supplies during his voyage. Lindemann's own observations about reactions to scarce fresh water supplies became the basis for the World Health Organisation's navigation recommendations.
Bombard died in the southern French town of Toulon in 2005 at age 80.