When you say "a trip to the beach", which do you mean?
The ocean & gulf beaches are not considered to carry much risk because the saline water is inhospitable to NTM/MAC.
That said, many of us in this group spend time on or near fresh water every year. I spend a lot of time paddle boarding & playing on Midwestern lakes all summer. I take the single precaution of keeping my face out of and away from the water. That means no tubing, diving, etc, but I look forward to pontoon boat rides and picnics on the shore. (Hard not to explore the underwater with my littles, so occasionally I will don a mask and snorkel for a quick look.)

I am sure there is some risk involved, but since NTM/MAC is everywhere - air, water & soil - it is impossible to be completely safe. I just stay conscious of high risks & take precautions - like masks when I garden, and keeping the soil moist to reduce particles in the air.

AND I definitely continue 7% saline nebs and airway clearance to make my lungs inhospitable to MAC. Written as I puff & cough!

What level of risk do you find to be acceptable? Have you asked your doctor about visiting the lake?
Sue