Marathon Swimming is defined by distance. Marathon Swims are categorised as anything equal to or greater than 10KM. There are hundreds of events worldwide for the marathon swimmer. Sea, rivers, lakes and lochs all host swimmers going the distance. Events can be mass participation of elite with many catering for both.
The pioneers of endurance. The first modern day marathoners were not runners. So, despite marathons being long associated with running, arguably the first modern day marathoner was Capt. Webb. Decades ahead of his time and ahead of the sport of athletics with his marathon swim across the channel.