Ultra marathon swimmer
noun - Ultra marathon swimmer is an athlete who attempts and completes solo open water swims that is defined as any swim longer than a marathon swim (distance of 10 kilometers or 6.2 miles). However, different interpretations of the minimum distance of an ultra marathon swim include a minimum of 15 kilometers, 25 kilometers and 20 miles (the distance of the English Channel and Catalina Channel).
10 kilometers is the distance that is defined by the International Olympic Committee, FINA, many national swimming federations and the International Marathon Swimming Hall of Fame as a marathon swim. An ultra marathon swim is longer than a marathon swim.