Fish need an air hole in frozen ponds so they can breath.

Once all the leaves have fallen, prepare your pond for winter. Don’t drain the water from ponds as it will only fill up with debris, rain and snow during the winter. Remove fallen leaves and other organic matter as you can. Use a pond skimmer net to scoop up the debris. Remove potted pond plants and cut them back to their base. Tropical plants need to be removed and discarded or overwintered indoors. Don’t worry about water lilies as they are quite hardy. 

Maintain a balanced pond ecosystem by adding Cold Water Beneficial Bacteria. This aids in breaking down organic debris and any fish waste. This lessens toxic gasses building up in the water. Turn off the pump and clean its filter. Once done, put the plants and pump at the deepest part of the pond.

For ponds in mild climates keeping the pump running or using a pond bubbler not only aerates the water, it prevents the pond from freezing over. For cold climates, disconnect pumps and check valves then drain from all the plumbing including any waterfall.

If your pond contains fish, add a fish pond heater. A floating pond de-icer keeps a hole free of ice, which is important for the survival of the fish and the pond as it prevents harmful gases from being trapped under the ice. Winterizing fish pond kits are available on Amazon, pet and aquarium stores as well as places that sell ponds. Finally cover the pond with netting to avoid debris from collecting. 

Waterlilies are hardy and don't need to be removed from the pond during winter.