I purposely tested this out in a storm we had a few weeks ago. I had the range to myself so I kept the gun out (in my hand) while walking the range to check targets/repaint steel as well as kept it out in between drills and obviously for the drills themselves. Gun was exposed in a heavy downpour for about 45 minutes, with a primarily muzzle down (unless on target) orientation, and I did not have any issues using the dot.
Edited to add: FWIW, the gun is a CZ P07 with an RM06 installed by L&M Precision.