The only conditions under which I would ever consider doing this, is in a life and death situation. This is illegal in all 50 states, so I do not recommend you try it. But if the situation is desperate, here is a way to catch ducks, geese or swans. Diving ducks and sea ducks dive deep underwater to get food. Dabbling ducks feed on land or on the surface of the water. First of all watch the ducks and see if they are diving ducks or dabbling ducks.
Most common water fowl feed on a diet of grasses, aquatic plants, small fish, insects, worms, small amphibians (frogs and toads) and small mollusks. Dabbling ducks or water fowl that feed on land or the surface of the water are the easiest to catch. All you need is a good strong fishing line (10 Lbs test) and a treble hook. Bait the hook (bread works well) cast out near the ducks and wait. When the duck bites give him time to swallow the hook and pull him in. Have something ready to wrap him up in. It makes them easier to control until you can dispatch them. Again I repeat this is only something you should ever consider doing if your life depends on it.