I was a swimmer for much of my life, and like the poster above--you don't have a choice. You either hold it the whole practice, or like everyone else, you go.
I had a friend, who was a little crazy, who would use peeing as a defensive manuver when playing water polo. If she was defending someone who looked a little "girly;" she'd get really close to her ear and whisper "I'm peeing on you, I'm peeing on you" in kind of a sing song voice.
I have no idea if she would actually go, but it was hilarious to see the girls swim away as fast as they could