The information about the 2012 London Olympics holding the Olympic 10K Marathon Swim in the ocean was CONFIRMED TO BE FALSE!
Key individuals involved with the planning of the 2012 London Olympics confirmed the preference of the Serpentine in the heart of London as the intended site for the Olympic 10K Marathon Swim.
The Serpentine was man-made by joining a series of small ponds together back in 1730, under the direction of Queen Caroline. One of the first competitions in the Serpentine was in 1837 promoted by a London wine merchant by collecting an entrance fee from competitors. A gold medal and the title of 'Champion of the Serpentine River' was first awarded in a 1838 Grand Match. Twelve athletes, including the Champion of England and the Champion of London, competed in front of 20,000 spectators over a distance of 1000 yards in a Bridge-to-Bridge race.
In 1872, J.B. Johnson, the Captain of the Serpentine Club and reigning Champion of England, was recognised as the first man to attempt the English Channel (later successfully crossed for the first time by Captain Matthew Webb in 1875). Johnson, beat A. Truatz, the reigning American champion, in Long Branch, New Jersey in 1874 to become the Champion of the World, winning US$2,000 and a Tiffany cup.
Upper photo of Lake Serpentine and the Serpentine Swimming Club members is from the Serpentine Swimming Club website.
Lower detail is from The Graphic from September 7th, 1872 and shows J. B. Johnson starting his English Channel swim attempt on August 24th 1872, courtesy of the Dover Museum.