I suspect it was a cob and not a swan. In the 1920’s a man in Methuen, Massachusetts drown in shallow water after being attacked by a cob defending cygnets. (The man must have acted in a manner that appeared to be threatening to the cob.) Swans are BIG and attack by beating their victims with the elbow joints on their wings. They can inflict serious injury.
My son-in-law was fishing in the Charles River in Natick, when he glanced to his right and noticed a cob standing on tiptoes next to him raising his wings menacingly. My s-i-l got the message and backed away.
Swan, cob, cygnet. Time to go to Google I guess.