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Open water swimming takes place in outdoor bodies of water such as open oceans, bays, lakes, rivers, canals, and reservoirs.
The beginning of the modern age of open water swimming is sometimes taken to be May 3, 1810, when Lord Byron swam several miles to cross the Hellespont (now known as the Dardanelles) from Europe to Asia.[1]
In the first edition of the modern Olympic Games in Athens in 1896, the swimming competition was held in open water. In 2000, the Olympic Games first included a triathlon with a 1500 m swim leg, and in 2008, a 10 km open water swim. 5, 10, and 25 km open water races are included in the General Fina World Championships.
The activity has grown in popularity in recent years with the publication of bestselling books on "wild swimming" by authors such as Kate Rew and Daniel Start.

Source:  WIkipedia

http://www.10kswim.com/learn.html#article01    It is also known as marathon swimming,,,,However if you swim in outback Aussie,,you are automatic Crocodile Food,,and in some places "the ocean"..>>>>>>><<<<<<<..

Around here we call it shark bait

Too risky for me,  I'm not a  particularly good swimmer. 

Skinny dipping?