Courtesy of WOWSA, Huntington Beach, California. The largest 10 km marathon swim in Japan is between the island of Enoshima and the main island of Honshu: the Shonan Open Water Swimming in Sagami Bay, Kanagawa Prefecture, Japan. In 2016, there were a total of 450 entrants in the race: 371 men, 77 women and 2 disabled athletes). 431 people started the race