Well, while I don't personally do it, there's nothing unhygenic about it. Urine is pretty much completely sterile water with some various salts (which are the cause of the smell/color associated with urine) mixed in varying concentrations depending upon how much water one drinks. If you were desperate enough (like stranded in ID in the middle of nowhere without clean water available), drinking ones own urine is advisable. I'd rather drink my own urine than drink water out of a river or stream, which usually have animal fecal matter in them, which is extremely unhygenic.

As to why someone would urinate in a pool, the simple answer is that it's convenient. Can't really think of any other reason. If one thought that urine was really dirty, I suppose one could either be aroused by the idea of contaminating the pool or be spiteful if they're not happy with the hotel. But since urine, unlike feces or vomitus, is far too acidic to harbor any sort of pathogens, this would be a mistaken belief. In fact, the urigential area is the most clean part of ones body.

As for the actual questions you pose, I have no idea. I couldn't imagine it being more than 5 percent, if that.