I have never hunted ducks with a bow, but this might be helpful: http://snaro.net/bowhunting-ducks/
I really like the snaro spearo heads.
In my opinion, it would be easier to get them while on the land or water, although that seems like shooting a deer at a feeder-slightly unethical. If you're good enough, go for the landing shots. If you can hit them while they are flying over or taking off, you're an excellent shot.