My understanding is that it's the water aerosols that can expose us to MAC/NTM. Outside swimming pools are less risky than indoor ones, as there is more air circulation outside than inside. MAC/NTM is chlorine resistant and everywhere. It's a colonizer not a pathogen in water.

Consider reviewing https://www.ntminfo.org/wp-content/uploads/2019/06/NTMSupplementalGuide.pdf