Kids were terrified that if they peed in a swimming pool, a chemical dye would make a visible cloud or change the water colour, exposing them as the culprit.
No such dye exists in standard swimming pools. This myth was created as a deterrent to keep children from urinating in pools, and it worked remarkably well. While some high-end pools might have added dyes as a prank or gimmick, they're not standard pool chemistry and most don't work as intended. The real reason not to pee in pools is hygiene and courtesy.
Reception
Sources
- Snopes: Pool Pee Dye PRIMARY
- CDC: Pool Chemistry REFERENCE
- Wikipedia: Chlorine in Pools REFERENCE