It is best to give your ducks a fenced area -- hawks and eagles can attack ducks on a pond, owls can be an issue at night, coyotes can swim, as can stray dogs. You have no way of telling the ducks to STAY on the pond all night - they forage for food at night and it is quite possible that they would be hunting for goodies at the wrong time . . . .
You can try it out and see -- if there isn't a high predator population you may be okay (but I'd expect losses) I am positive that without our electric fence, we would have lost all of our geese / ducks to roaming dogs, eagles, and our large coyote population. The wild ducks seem to make do, but I think they have better instincts than domestic ducks - plus, there is no telling how often the wild ducks lose mates or babies to predators.
I think most people on here have a fenced area for the ducks, and let them free range with supervision during the day. If the pond is a large one, you may have some issues getting the ducks out of the pond and back to their house in the evening though, but if they are used to being fed, they'll get in the habit quickly. Mine drop whatever they are doing if they see the feed scoop!