No matter what your parents told you, there isn't any magical chemical that when added to a swimming pool will reveal the presence of urine in the water. As "Alan" at the Aqua Clear web site says: "There is no chemical that can function as an indicator for urine in a pool." Others in the industry concur this belief is all chimera and no substance.
Those in the pool supply business are routinely confronted with requests for the "urine-indicator dye" (as the mythical substance has come to be known). The belief in such a chemical spans the United States, Canada, and Spain, as does the juvenile certainty particular pools are spiked with it.
Experts on such matters say although a reliable dye could be produced, the trick would be getting it to react only to urine and not trigger in the presence of similar organic compounds likely present in swimming pools. ... Moreover, kids are kids their expected reaction to the news that pissing in the pool would produce bright purple or red trails would be to jump right in with the intent of putting that theory to the test. Especially in a public pool where one's indiscretions can be blamed on the fellow swimming by, what kid wouldn't avail himself of the naughty pleasure of invoking billowing clouds of dye?
Or, as one old-time Boston-area poolman put it, "If such chemicals did exist, every municipal pool in Boston would be bright purple." (A heartening thought, that. One could drown Barney, and the body wouldn't be found for days.)
Chalk this belief up as what it is: yet another sneaky parent trick meant to keep the kids in check. ...
What I find funny is the water difference between a normal toilet and a pool is so great that a tiny amount of pee isn't going to make a pool like a toilet. Now if you drop a "log" in the pool then that is a TOTALLY other story - 
I did the same thing years ago when half the neighborhood swam in our pool. I heard one of the little boys saying he wouldn't get out of the pool to pee. So I told him if anyone pees in the pool, the water will turn bright red around them. The look on his face was priceless. Every once in a while I would overhear the kids telling other kids about the "red water", so I stuck to that "fact" for years. 
