If an egg is not under the mother for a significant amount of time and, especially if it is in the water, there is no chance that the egg is viable. Mother swans will dispatch eggs by taking them into the water or kicking them from the nest if they are no good. So, this is a non-viable egg. You need to put it back where you found it because it can make a very big mess with an obnoxious odor when it eventually explodes. The Regal Swan