Even fairly small pools have thousands of gallons of water, a full bladder holds about 2 cups. You'd have to have 80 people empty full bladders in a 3900 gallon pool(a 12' round above ground pool) just to get to .25% piss.
Once, recently, was locked out of the room anyways (didn't know that yet tho)... There's plenty of chlorine to handle any bacteria and afaik I've got a whole lot of nothing. Plenty of kids do it anyways. Just uh... Not my proudest moment as an adult but I usually hold until I gotta go and my partner was out and about trying to figure out the room situation with the keycards, sooo not gonna worry about it too much.