If water can find a way, it’ll cause the gears to rust. bmoscato’s suggestion is good.
The ideal way would be to use a shaft seal and O-ring.
This having been said, the Lynxmotion A4WD1 uses the same motor and that section is exposed. It’s used outdoors, and we have not heard of any issues with water infiltration, though we would certainly not suggest it be used in the rain or wet areas.