Your fish will be hibernating in the deepest part of your pond to stay warm, so it's better to place equipment in a shallow region. Water divides itself into distinct layers with the warmest water settling at the bottom. Disrupting this layering can cause trouble for your fish, including drawing them out of their semi-hibernating state and stressing them. You're right, though, that it's important to ensure that some surface of your pond maintains contact with the open air and allows harmful gases to escape and oxygen to enter the water.
De-icers help to keep a hole open in the ice during winter months, and you may be able to install a timer to run a de-icer for an hour or two a day and keep things at equilibrium. In a pinch, a hole can also be re-established simply by pouring hot water over the thin ice. If your weather is particularly severe, an extreme cold snap is in the forecast, or you're going to be away for a few days and unable to check the pond, a de-icer is definitely the way to go.
