Marathon swimming is a type of open water swimming characterized by long distances and following traditional rules based on swimming the English Channel (where some of the first marathon swims took place).
While running marathons have specified distances, marathon swim distances will vary. However, the common minimum distance of a marathon swim is defined as 10K (6.21 miles) which is the distance used in the Olympic marathon swimming event and accepted by many swimming organizations.
The above is FINA’s definition of marathon swimming which states that a marathon swim is 6.2 miles (10K) in length; the Olympic 10K Marathon Swim being its most widely known race. While FINA (as well as many other swimming organizations) defines marathon swimming as 6.2 miles (10K), a few swimming organizations, such as the US Masters Swimming and USA Swimming, have no definitions for marathon swimming.
While today 6.2 miles (10K) is the widely accepted distance for marathon swimming, the early days of marathon swimming, circa the 1960s, saw marathon swim distances of at least 10 miles (16K) due to the relative times involved in completing it. Additionally, other literature shows some regions define marathon swimming as any open water swim that is over 1500 meters. And, yet, others define marathon swimming as open water swimming longer than 10K.
Much like with running marathons vs ultra-marathons, in swimming, an ultra-marathon is typically any swimming event over 6.2 miles (10K). Though this is the widely accepted definition there is some debate over whether the minimum distance should be anywhere from 15K to 25K.
On a side note, we recommend that you check out our post Can You Do Too Much Swimming? How Much is Too Much?! (Age & Experience)
How Long Does A Marathon & Ultra Marathon Take to Complete?
In addition to distance, marathons & ultra-marathons -running and swimming- are also defined by how long it will (roughly) take one to complete [it]. Swimming marathons are not only defined as being 6.2 miles (10K) in distance but also by being able to be completed in 2 to 6 hours- the exact time it takes one to complete a swimming marathon will depend on their swimming level/athleticism and training (as well as nutrition). An ultra-marathon swim is defined as taking longer than 6 hours to complete (again, exact times will vary and be dependent on one’s swimming level/athleticism and training).
While 6.2 miles (10K) has been the widely accepted distance for marathon swimming, and, thus, ultra-marathon swimming is anything over 6.2 miles (10K), recently, the International Olympic Committee (IOC) decided to increase the distance.
Starting in [the] 2024 [Olympics] marathon swims will be defined as swims of 26.2 miles (42.2K). The decision to change the distance is meant to be a parity in distance between a running marathon and a swimming marathon. With this increase in distance will come an increase in time to complete the event.
That said, where a marathon swim of 6.2 miles (10K) would take one 2 to 6 hours depending on their level of training, a marathon swim of 26.2 miles (42.195K) will take, on average, 8+ hours to complete. Therefore, one can reasonably deduce that the completion time for a marathon swim of 26.2 miles will be 8 to 12 hours depending on one’s athleticism, training, and nutrition. And, an ultra-marathon, thereby, will logically take 12+ hours to complete.
Though everyone seems to have their definition of what a marathon swim is and how long it should take to complete as well as a good reason for their definition, FINA and other swimming organizations currently define marathon swimming as 6.2 miles (10K) in length and being able to be completed in 2 to 6 hours. Ultra-marathon swimming, as defined by those same organizations, is any swimming event longer than 6.2 miles (10K) and taking longer than 6 hours to complete.
[Note: come 2024 this definition will change to marathon swimming being defined as 26.2 miles (42.2K) with a [suspected] completion time of 8-12 hours and ultra-marathon swimming as anything over 26.2 miles with a completion time greater than 12 hours.] (Source A)(Source B)(Source C)(Source D)