A marathon swim is a swim of at least 10 kilometres / 6.2 miles.
Swimmers can participate in marathon swims in the following ways:
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Signing up for a swim being managed or overseen by a swim organization, for example the Catalina Channel Swimming Federation for swims across the Catalina Channel. A swim organization may have their own rules, or use those published by independent ratifying bodies such as the Marathon Swimmer's Federation (MSF) or World Open Water Swimming Organization (WOWSA).
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Planning their own swim route and submitting details and documentation afterwards to an independent ratifying body such as the MSF or WOWSA.
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Planning their own swim route and not seeking ratification.
The "ratification" of a swim means when a swim organization or ratifying bodies such as the MSF and WOWSA reviews the details and documentation of a swim after it is over and authenticates that the swim's official rules were indeed followed. The purpose of ratification is to validate that any swim following that specific route has been conducted against the same standards, regardless of who the swimmer is.
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Swim organizations and ratifying bodies generally agree that an "unassisted" marathon swim must be done without any significant propulsion, contact, support or performance-enhancing aids and also requires the swimmer to remain in the water from start to finish (with the exception of relays, or course). The swimmer is permitted standard equipment that includes a support boat and pilot, a kayak and paddler, an official observer, the swimmer's feeds and standard attire such as a fabric bathing suit, bathing cap, goggles and other items listed here in Section 1.3 Rules - standard swimmer attire. Note that not all swim organizations use kayaks and paddlers.
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In contrast, an "assisted" swim is a swim that makes use of anything other than whatever the swim organization or ratifying body deems as suitable for unassisted marathon swims. This could include such things as a wetsuit, fins, tow float, watch for pacing, underwater streamer lights for guidance, a shark cage, or water with a swift current that delivers a significant speed advantage.​
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It's important for the swimmer and their boat support crew to know the official rules of their swim, so they both can ensure the rules are carefully followed. Even if the swimmer is not seeking ratification, the swimmer should still tell their crew what lines they do not want crossed in the conduct of the swim. This section of the crew guide will cover some of the basic principles most commonly used in official rules for unassisted marathon swims further down below, as these rules are specific and unwavering.​​
1.2 How a marathon swim works
It all starts when the swimmer commits to a big hairy goal. Maybe they grew up always wanting to swim across that big lake in their hometown, were inspired by a childhood friend who swam around Manhattan island, or stayed up all night tracking Sarah Thomas' 4-way crossings of the English Channel. They're inspired and motivated, apply for their swim, pay their fees, train really hard, complete and submit any qualifying swims required before the big day comes around.​
The swimmer can sign up for a solo swim, a tandem swim, or a relay swim:
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Solo swim: the swimmer undertakes the entirety of the distance from start to finish on their own. The swim ends when either the swimmer exits the water at the finish, or the swim is abandoned early for any number of reasons such as the swimmer feeling unwell or no longer making forward progress, bad weather, boat problems.
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Tandem swim: the swimmer undertakes the entirety of the distance side-by-side with another swimmer of equal speed. Both swimmers are expected to finish together, but if one of the swimmers abandons the swim, the other swimmer is usually permitted to finish the remaining distance on their own.
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Relay swim: the swimmer is part of a team of swimmers, who each swim for a segment of time and in a pre-determined order. The swimmers are expected to keep cycling through in order, but if one of the swimmers opts out of a segment or abandons the swim, the other swimmers are usually permitted to finish the remaining distance without them.
Stephen Rouch, Manhattan Island 8 Bridges 2017
A marathon swim could be scheduled to begin at any time, day or night, depending on what how the swim organization runs things. The start usually happens in one of a few ways:
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The swimmer and crew drive by car to the shoreline and the swimmer waits for the boat/observer’s official start signal before walking into the water and starting to swim.​
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The swimmer and crew get on the boat which takes them by water to the swim start, the swimmer jumps off the boat into the water and swims to the shoreline, exits the water and waits for the boat/observer's official start signal before getting back into the water and starting to swim.​
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If there is no safe access to the shoreline, the swimmer may touch and release some part of the natural shoreline (e.g. cliff face) once the boat/observer's official start signal is given.
The swim ends when the swimmer exits and fully clears the water, or where that's not possible, touches some part of the natural shoreline (e.g. cliff face) at the finish. ​
The swimmer is accompanied by a support boat and possibly a kayak and paddler as well. The boat will have one or two pilots on board who navigate the boat, an official observer who monitors that the rules of the swim are being followed, and the swimmer's crew who deliver feeds and medication, provide motivation, and monitor the swimmer's well-being. If a kayak and paddler are being used for the swim, the paddler will be the one delivering the feeds and medication to the swimmer instead of the crew on the boat. The boat may be positioned alongside the paddler and swimmer, but in rough winds or strong currents the boat would have less agility and control than the kayak would and may move off to the front of or behind them. On some swims that use kayaks, the swimmer communicates exclusively with the paddler and may never even speak with their crew on the boat unless something goes wrong and support from the boat is needed.​
Boat support only:
Shannon House Keegan, In Search of Memphre Double 2025
Boat and kayak support (video taken from boat):
Robin Batchelor, Manhattan Island 2025
Throughout the swim, the official observer - and possibly the swimmer's crew - will also complete a log of important details about the swim, take photos and videos of the swim, and gather any other documentation required by the swim organization or ratifying bodies for their review and authentication that the swim's official rules were indeed followed.
1.3 Rules - standard swimmer attire
The swimmer is permitted the following:
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One swimsuit made of porous, textile material. Not below the knee, onto the neck, or beyond the shoulder.
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One bathing cap made of latex or silicone, no neoprene.
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Goggles, earplugs, and noseclips.
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Sunscreen and grease (includes zinc oxide, bag balm, etc).
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Safety lights for nighttime visibility (see Section 10 Crewing in the dark).
Justine Brousseau, Catalina Channel 2021
1.4 Rules - no contact
The best way for crew to be sure they are adhering to the no contact rule is to (a) not make direct physical contact with the swimmer in any way and (b) not let the swimmer touch the boat or any other thing that could possibly support them. Follow those two principles, and you should be good.
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If you're new to marathon swimming or crewing, this means...
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- You can't throw the swimmer a pool noodle to rest on for a bit when their arms get tired
- If you get in to support swim alongside the swimmer (see Section 12.3 Support swimming), you can't give them a proud hug first
- If there's a sudden thunderstorm and the swimmer gets on the boat for a few minutes, the swimmer can't get back in the water
1.5 Rules - no drafting behind or alongside the boat
The swimmer is not permitted any benefit from drafting behind or inside a vessel's bow and displacement waves, except when feeding (quoted from MSF). Unless your swimmer is a super speedster, chances are the boat is moving slowly anyhow, but crew should still keep an eye out to make sure the swimmer's placement can't be called into question.​​​​​