@payette78 & @dstella - It becomes a question of what we know, and what our personal risk tolerance is.

We know marshes, whether fresh water or brackish, tend to harbor a lot of NTM. We know that with Bronchiectasis and/or MAC we should avoid sources of concentrated bacteria. Thus we know that repeated, unprotected exposure is a risky activity. I know my tolerance for taking risks is higher than most people, but I don't know yours.

Given all of that, what would I do? If I was on a trip or excursion that included a one time hike near a marsh, I would wear a new N-95 mask and discard it at the end of the outing. If it was a marsh near my home (I have a couple) I would choose a safer path.