If you live in a seasonal climate, then using pond heaters during the winter is likely necessary. Also known as de-icers, the purpose of pond heaters is not to heat the pond and thereby keep your fish warm. Rather, they are meant to melt a hole in the ice, allowing toxic gases to be released.
Floating heaters simply float on top of the water, and when the water begins to ice over, the heater automatically turns on, keeping a hole in the ice. There are also submersible heaters, but use more energy.
Do not pound on the ice or drill through it to create a hole. This can be bad for the fish. Also consider an aeration system which can be used all year around to provide additional oxygen in the pond and will keep an opening in the ice all winter.