20 Ga. is fine, just live with the limitations of a smaller shot load than a 12. I have not hunted ducks in years, but, I took many as a child with a .410 and 20ga; the ducks could not tell the difference.
Please do pattern your gunswith the type of shot you intend to hunt with before you go out for the first hunt; you may have to try a few different loads or choke combinations (provided you have screw in chokes) to find what works for you. Maybe try a few rounds of skeet.