Could you please post what type of bird bath heater you use. This is my ducks first winter in Kentucky, so it doesn't get AS cold, but I still want warm water for them to swim. What type of "tub" did you use with it?
A picture is worth a thousand words...
Have you tried looking around for a heated Five-gallon water bucket? I've used that for a few years and its worked great, I just sink it into the dirt in their run to its flat on the ground. It's big enough that the ducks will take turns bathing in it.