wild swimming       
noun [ U ] UK uk/ˌwaɪld ˈswɪm.ɪŋ/ us/ˌwaɪld ˈswɪm.ɪŋ/
 野泳,指在非正规场所游泳戏水,例如在河道、湖泊、海滨等地方而不是在泳池
swimming for enjoyment in rivers, lakes, the sea, etc. , rather than in swimming pools
  Cold water, possible currents, no lifeguards: wild swimming is more dangerous than an indoor pool. 野泳比室内游泳更危险,因为水冷,可能会遇到激流,也没有救生员。 
 - More examples
 - Deakin first reached a wide audience with his journal of wild swimming, Waterlog.
 - The Scottish Inner Hebrides are about as wild as wild swimming gets.
 - The hotel has wild swimming lessons on a three-night break.
 - Wild swimming: Forget smelly indoor pools, and dive into Britain's cool, sun-kissed rivers, lakes and sea.