It will need to be caged so it can not make contact with the plastic sides an melt. Ducks will chew on power cords. Run it through some PVC pipe or take an old hose slit it lengthwise and run the cord inside.
If you reduce the air contact with the heated bucket you can improve it's efficiency. Take a larger bucket like a 5 gallon pickle bucket. Drill a hole on the side to run the cord through and a few drain holes on the bottom. Place a non flammable insulation on the bottom and sides, bubble wrap silver stuff would work well, put the heater bucket inside. Raise the bucket on bricks or dirt so the tops are even. Then take the lid for the 5 gallon bucket, cut a 4-6 inch hole just big enough for them to dunk in. Snap the lid on and let it snow