spraying anything on an ar6400 would not be a good idea as the resistive strips for the servos' feedback will get coated and mess up the movement. it would be much better if you could tape up the hatch to keep the water out of the hull. only other option would be a separate receiver like a Deltang or one of the HK oranges. then you could coat it in waterproofing, or a small balloon etc.
it does look like it goes very well, must have a good power to weight ratio