We are assuming that when you mean that the eggs were upended in the water for a day, that the entire egg was not submerged, just the bottom of the nest. If they were completely submerged, the forming cygnet would not have an air supply and would drown. So, if this occurred, the chances of survival are nil to none. However, if there was a portion of the eggs that had no water and oxygen was still available, then the eggs could be viable.

It also must be noted that swans and other birds/wildlife know a lot more than we do. The swans can tell if the eggs are still viable or she would not have returned to sit on them. Now, having said this, there is still a chance that if something is still alive, it may not survive from the trauma of being upended. Please let us know how this progresses as it will be interesting to know how many actually survived. The Regal Swan