Difference between "游泳" and "游水"
游水 is mainly a colloquial Cantonese term  http://www.cantonese.sheik.co.uk/dictionary/words/1332/


  游水
  
  /jau4 seoi2 /  (jyutping)
  
  swim, swimming. Mand.: 游泳
  
  the entry of 游水 doesn't even include a Mandarin pinyin for it


Also logically, you can swim (游泳) in any liquid ; while 游水 specifically means swim in water, but even Cantonese ignore it, since it is very rare that someone swim in a liquid other than water