I would think that the spinning hubs of the propellers would do a pretty good job of keeping the moisture out of the motor. Plus, unlike a brush motor, these are run inductively....there are no open exposure points in the motor. I flew, a few times, in dusty conditions where upon inspection there was a fair amount of dust that had settled in the motors. After using compressed air, there was still a layer of dust. Using fresh water, I washed out JUST the motor and then blew it dry. Clean as a whistle and no problems afterwards. Your mileage may vary....
...Jim
...Jim