Brushed Motors are all waterproof (its the stuff in the water that can wreck you motor such as salt) but brushless are not waterproof unless the manufacturer says so because they seal it or something. receivers and servos can be wateproofed by using a balloon but I'm not exactly sure what are the steps. Someone correct me if I'm wrong.
Doesn't the water wreck your ball bearings in your car?
I don't recommended to submerge the electronics completely in water even if they are waterproof. Maybe someone who owns waterproof electronics can chime in..