u/-LsDmThC- gave a good answer, although fluoride is usually there as a health measures to keep teeth healthier. Chlorine is also very often used in municipal water systems.
That being said, outbreaks do happen, for an example see: https://arstechnica.com/science/2022/08/fecal-fountains-cdc-warns-of-diarrheal-outbreaks-linked-to-poopy-splash-pads/
ETA: Here’s another one: https://arstechnica.com/health/2023/06/its-summer-and-that-means-disturbing-swim-advisories-heres-our-top-5/ since the first linked discussed areas where regulations may not require the use of disinfectants in water features.
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