Thank you - but recently, he said on NTMinfo.org, that outdoor pools are safe to use, he did not say they were free of NTM. His rationale was that the high rate of dilution of any NTM in the rising vapors made them an unlikely source of infection.
Here is what he said:
"First, outdoor streams, rivers, ponds and lakes will have aerosols with NTM, but the NTM cells are rapidly dispersed by the great volume of outdoor air. It is necessary to judge whether the risk of aerosolization and inhalation is high or low. Outside the risk is low; inside (indoor spa or indoor pool) the risk is high. Remember, infection is due to a combination of two factors: number of NTM in aerosols and duration of exposure. Sitting by a lake for a short while is probably safe, but sitting by a lake all day may put you at risk." (https://connect.ntminfo.org/discussion/evaporation-from-rivers-and-lakes)

I avoid all indoor pools, saunas, steam rooms and hot tubs because you can see the accumulation of droplets in the air. For similar reasons I avoid soaking in outdoor hottubs, but I do swim or exercise for short times in an outdoor pool or lake, then leave the immediate area.