The answer to this question would depend on the predator, it's size and the circumstances of the encounter.

For instance, if it is a small predator near the banks of a waterway and if the swans felt immediate danger, to their young and could not readily flee, they could strike at the predator and even try to pull it into the water where they would have an advantage over a land mammal. If there was ample time to flee and no young, they would probably fly. Again, this totally is determined by the circumstances such as protecting territory, nesting sites and young.