You might like to read this discussion about lakes: https://connect.mayoclinic.org/discussion/water-at-freshwater-lake/
I go to the lake all the time with my family, and I have never even considered a mask, even while I was being treated for MAC. I don't put my face in the water unless I fall off the paddleboard. We paddleboard or canoe, so we are not stirring up and inhaling a lot of spray or mist, and we go only to quiet lakes so others are not stirring things up around us. Remember all that fresh air really dilutes anything you might breathe in...
If you are going to the ocean, you can relax even more, the salt water in oceans doesn't support the growth of MAC.
I am maybe a little less stringent than some other members, but I have lived safely without reinfection for over 3 years by being cautious in high risk situations (hot tubs, indoor pools) and less so in lower risk. You must decide what works for you.
Sue