Open water swimming is one of the five aquatic sports governed by Federation Internationale de Natation de Amateur (FINA). Swimming, water polo, synchronized swimming, and diving are the others within FINA's mandate. Open water swimming is defined by FINA as any competition that takes place in rivers, lakes, or oceans. Open water swimming is also an important part of both the Olympic triathlon (swim/cycle/run segments, with the swim 0.9 mile (1.5 km) in length) and the longer version, the Iron-man, which has a 2.4 mile (4 km) open water swim.
FINA sanctions two forms of open water swimming. Long distance swimming is an open water event with a maximum distance of 10 km (6 miles); marathon swims are any races of a greater distance. The open water world championships are sanctioned for distances of 5 km, 10 km, and 25 km. The FINA rules regarding the race venue are uncomplicated, with the chief requirements being a race time water temperature of at least 16°C (60°F), and a minimum depth on the course of 1.4 m (4 ft 8 in). FINA championships may be conducted in either salt or fresh water.
Open water swimmers are prohibited from using any device which may aid in their buoyancy or propulsion, including wet suits. A wet suit, if constructed from materials such as polypropylene, will add as much as 5% to the buoyancy of a competitor. Grease or other similar products may be used in a FINA event; such materials are often employed by open water swimmers to provide an extra layer of insulation to the swimmer's body in cold water.
Aside from the competitive issues relating to open water swimming, the activity is often described as having the type of relationship to swimming in a pool that trail or cross country running has with respect to track. Open water swimming permits the swimmer to move without the boundaries imposed by a pool and the consequent interruption of the swimmer's stroke and rhythm.
Successful open water swimmers, both in those FINA styled disciplines and the triathlon swims, place significant training emphasis upon efficiency, and the corresponding ability to conserve energy and maintain an even pace. The most important variables to be considered in open water venues are the potential impacts of waves or currents.