Some sports make this easier than others. Swimming, of course, is the easiest of all. Like your gross nephew, pro swimmers pee in the pool all the time. Human dolphin Michael Phelps has admitted to it, and so has Ryan Lochte, who said, "When we're in the water for two hours, we don't really get out to pee. Chlorine kills it so it's not bad."
There's a similar deal with golfers, who have plenty of bushes and other greenery to nip behind to water the fairway. Golf legend Ernie Els recalled that he "had an emergency one year at the Heineken Classic, and they have those [sponsor] signs around the tees, so I just walked behind those signs and went. Just had my hands on my hips, and there I went."
But athletes in other sports need to get a lot more inventive to discreetly drain. Between the 20 pounds of strapped-on gear and the high-speed camera lenses, NFL players can't whip it out and answer nature's call. So these jocks have to come up with inventive ways to go, including peeing in a Gatorade cup, having a buddy hold up a towel as you pee in a Gatorade cup, and if you can't find a Gatorade cup, simply peeing your pants and hoping it dissuades tackles.
But the true masters of the sneaky pee have to be long-distance runners, all of whom have a strategy for going with the flow. Women who want to avoid the tactical squat often wear a small diaper or extra-thick menstrual pad to catch the runoff. Meanwhile, male veterans all learn to check their six, gauge the wind direction, and then snipe a pee off the track at full speed like the worst drive-by.