Swimming (游泳 “you2yong3”) is one of those words composed essentially of “swim-swim” to refer to the activity in general. However, the two characters break off to form rather interesting collocations on there own.
游 (which I can never remember how to write without looking at it) has a much broader semantic space than “swimming.” It’s original meaning has something to do with rivers, which can still be seen through the words upstream and downstream (上游 “shang4you2″ and 下游 “xia4you2” respectively), while its use in 游戏 (you2xi4 “game” and 旅游 lv3you2 “travel”) occur an order of magnitude more often than 游泳.
泳, on the other hand, pretty much exclusively related to swimming and is a sticky morpheme (meaning it shouldn’t be showing up alone). If you are watching the Olympics on CCTV Sports, you’ll see it show up in the names of the various swimming styles, such as 仰泳 (yang3yong3, face up-swim, i.e. backstroke), 蛙泳 (wa1yong2, frog-swim, i.e. breaststroke), and 蝶泳 (die2yong3, butterfly-swim, i.e. butterfly stroke); in swimming accessories, such as 泳衣 (yong3yi1, swim-clothes) and 泳帽 (yong3mao4, swim-hat); and for special types of swimming, such as the word of the day.
冬泳 (dong1 yong3, winter-swim) is literally what the hanzi suggest it means: swimming in the winter. It’s quite a popular activity in China, among old men, who swear by the daily ritual as a way to stave off colds.