Marathon swimming is a class of open water swimming defined by long distances (at least 10 kilometers) and traditional rules based in English Channel swimming. Unlike marathon foot-races which have a specifically defined distance, marathon swims vary in distance. However, one commonly used minimum definition is 10 kilometers, the distance of the marathon swimming event at the Olympic Games.
As in all open water swimming, tides, surface currents and wind-chop are major determinants of finish-times. For a given course, these factors can vary dramatically from day to day, making any attempt to draw conclusions about athletic ability by comparing finish times from performances undertaken on different days meaningless.