I have successfully overwintered a "temporary" pond in an 18" deep, 8 foot diameter plastic kiddy pool (plants, fish and all) in a minus 40 (oops that's celsius, not sure in farenheit?) winter. (Because we were moving and I had to dismantle my larger pool before winter). I did this by dropping a stock tank heater (with a metal guard to ensure that the plastic edges didn't melt) into the pond. It keeps the water just above freezing and keeps a hole in the ice, even in the coldest weather. I didn't keep track of what it did to my electric bill because I knew I needed it, so the pond wouldn't freeze solid. Then I built a raised pool (it was three feet deep but most of it was above ground except for the deepest parts in the centre) and I continued to use the stock tank heater with a lot of success. Everything survived through the winter. I purchased the heater at the local farm supply store.
The only problem with this idea might be the cost of running the heaters, but it certainly is something to consider for next winter.