I am not familiar with the device you describe. Nevertheless, certain things can be done to protect an electrical appliance from the weather.
If you can set it under the coop to keep it out of the direct rain, that would help even if it is not weather proof. Keep the connection of the power cord and any extension cord elevated so that it does not come into contact with water in a puddle. Make sure that the extension cord is plugged into an outlet that is protected by a ground fault current interrupter. Keep the exposed cord out of the path of foot traffic to avoid abrading it and damaging the insulation inside the cord. If you hang the cord, take care not to damage the insulation by draping it from nails or other such things. Position the waterer so that it cannot be turned over.
Hope this helps.