I'm sure the resort lifeguards must test the water at certain intervals (I was a lifeguard at a pool once) and add any chemicals to bring it to the proper levels. There are policies for this at all public type pools.
I'm sure pool water has a lot of things in it that we'd prefer not to be there.....frogs and bugs get caught in filters all the time, sunscreen and hair products are in the water, sloughed skin, leaves from plants, you name it....
Ever since I was a kid, I heard about chemicals that could be added that would show purple if someone peed in a pool. From what I understand now, that is an urban legend and does not exist. I've never heard of anyone actually seeing this happen either. But, as a kid, it sure did work as a good reason not to try
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