There needs to be some break in the ice so that they can eat and drink because they can choke on any food if they do not have water. They can also become very dehydrated and starve if there is no food. As long as they can fly and they are not physically frozen to the ice, they should be fine as they will find another area. However, if there is no water, then they can be accessed by predators walking across the pond. If you can break the ice to provide them with some water to drink, that would help. You want the water area to be large enough to stay unfrozen so that the swans do not fall through the ice (small area) and cannot get back out or they can drown.

Most swans have Down feathers which keep them warm on the tundra, so they should be fine as long as they are not frozen to the ice. The Regal Swan