Unfortunately, the fan has actual moving parts and is quite difficult to waterproof. However, I believe if you can safely remove blade assembly and put some conformal coating on the electrical parts thats the best you can do.
So, a while back, when I was first getting into RC's, one of the guys at my LHS told me about a product for waterproofing my RX's/ESC's/other electronics. It has stuck with me and has been a wonderous life saver. It is ACRYLIC CONFORMAL COATING. Non-heat cured. Meaning you don't need to apply...