Right now I have 12 ducklings that are two months old and they've been free-ranging on my pond since three weeks of age. The pond has gators, snappers, beavers and all kinds of life imaginable. Occasionally at night I hear them get spooked and swim away rapidly
The default response of a duck to predation is swimming away. I would imagine the opposite to be true if attacked from the water, perhaps they would go to the land
I don't think there will be many threats to a duck on water. Ducks even have a natural defense for birds of prey given that only half of their brain sleeps at a time, so they're always awake and ready