Friends of ours here in Colorado keep goldfish year round in a very small pond, much smaller than what you have photographed. It is approximately 18 inches deep and 7 feet across and has lily pads as vegetation. They have a floating heater in it that keeps a very small area free of ice in the coldest of our weather-0 degrees F. Their fish all make it without fail.
Maybe "The Pond Guy" offers these heaters?