Floating De-icers are needed in the Winter to leave an airhole in the ice of the pond’s surface. They are thermostatically controlled and are meant to leave an airhole in the ice, not to heat the pond.
Air holes are essential for toxic gases trapped under the ice to escape. Toxic gases are formed from decaying debris and fish waste. The gases are usually what kills the fish in the Winter, not the lack of oxygen. Cold water holds more oxygen than warm water.
De-icers are safe in all types of ponds. In many cases if you already have an aerator you may not need a de-icer unless you are in an area that gets extreme cold spells.