This is somthing i've always wanted to know, but was affraid to ask. That and i don't know any world class swimmers...LOL! YES THEY DO!!! 

Carly Geehr is a former member of the U.S. national
swimming team, and according to her, the answer is yes!  EVERY single elite swimmer pees in the
pool.

They rarely do it during races, because they’re too focused.  But during
warm-ups and practice, they do it all the time.  And they’ll do it before a race
sometimes too, while they splash water on themselves standing on the pool deck. 
Now you know.