I saw a video on YouTube where a researcher described a way to estimate the volume of pee diluted in the pool water measuring the artificial sweetener present in a sample.

This will exclude people that don't drink zero calories soft drinks tho...

Maybe there's a machine learning solution. If we could have devices that could somehow probe the temperature of the water on an X,Y graph and detect when the ambient temperature around an individual has changed we can sound an alarm to detect possible urination or even a laser that can draw a circle around the targeted area.