The mother swan's behavior seems to be indicative that there was a turtle or mink in the water and she was chasing it from the area and her cygnets. If it was a small mink or turtle, the mother swan could have handled the predator, safely chased it and kept it away from the cygnets with no problem. The cygnets were extremely safe on her back.

Water is the area that swans prefer in the event of a predator and most predators, including dogs, cannot stay afloat while being flapped with the heavy wings of a swan in the water. The Regal Swan