Water is densest at 39F, and that means the temperature will be nearest to the 39F range at the bottom of the pond. When the water is warmer, the warm water is on top, when the water is colder, the coldest water will be on the top. At depth, the soil temperature is pretty consistent, in the 50's, and will tend to provide heat to the bottom of the pond, if deep enough, throughout the winter, which sets up some micro currents of water warming toward 50, rising into water cooling from 32, mixing and 39F water falling back to the bottom. Once the entire pond is below 39, then the warmest water will be at the bottom and may not be 39, but a slightly lower temperature. Ice does reduce the evaporation of liquid water, lowering the heat loss significantly. A better protector than the ice is a lean-to, or dome structure covered with a plastic sheeting material, whether sheet poly, or the solar blankets for swimming pools, which provide a warm air space with near 100% humidity over the pond surface, reducing evaporation to near zero, and providing some solar gain during daylight hours.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard