Pool or lake, it makes no difference to Ous Mellouli.
As long as there's water involved.
The Tunisian won the gruelling 10-kilometre race at the London Games on Friday to become the first swimmer to win medals in both the pool and open water at the same Olympics.
Mellouli, who won bronze in the 1,500-metre freestyle last week, pulled away from a small group of leaders in the fifth of six laps and clocked 1 hour, 49 minutes, 55.1 seconds in the murky waters of The Serpentine in Hyde Park.
It was the second gold of Mellouli's Olympic career, having won the 1,500 at the 2008 Beijing Games.
After pulling himself up on to the makeshift finishing dock, Mellouli flexed his muscles, beat his chest and let out a few screams - then yelled again after getting the gold medal hung around his neck - marking Tunisia's first victory of the games.
Beijing bronze medallist Thomas Lurz of Germany took silver this time, 3.4 seconds behind, and first-time Olympian Richard Weinberger of Canada was third, 5.2 seconds back.
The 25-man field completed six laps of a 1.66-kilometre course in near-perfect conditions as fans lined both banks of the rectangular lake under bright sunshine.
The only previous swimmer to win medals in both the pool and open water at the Olympics was David Davies of Britain. He took bronze in the 1,500 free at the 2004 Athens Games and silver in the inaugural 10K Olympic race in Beijing four years ago.
Mellouli, who trains at the University of Southern California, is relatively new to open water swimming, but he also won the Olympic qualifier in Setubal, Portugal, in June.
Marathon races are usually contested in oceans or seas, where athletes have to deal with waves and currents - not to mention more wildlife - making this race relatively tame, and allowing Mellouli to use his pool expertise.
Mellouli stayed with the leaders for the first few laps then opened up a small lead on the fifth lap. That was all he needed to do, and it left three other swimmers fighting for the other medals, although world champion Spyridon Gianniotis of Greece dropped back in the final stages and finished fourth, 10.2 seconds behind.
Gianniotis had been among the favourites and was hoping to give Greece their first gold of the games after winning three World Cup races this year.
Lurz has won the 5K race seven times at worlds but has struggled to dominate the 10K event.
Meanwhile, the crowd favourite was Benjamin Schulte, a 16-year-old from Guam, who finished far behind all of the other competitors. Fans stuck around and applauded loudly when Schulte finally finished nearly 14 minutes after Mellouli.