I have some experience in this area, been duck hunting since I was 18, that was 39 years ago. I only have two preferences for waterfowling as it can be rough on your firearm, that is no wood, and the second is durability.
For your price range I have 3 suggestions, Rem 870, Benelli Nova, and Mossberg 500, all are pretty much bulletproof. I hunted with the 500 for 18 years and never had a problem. My only recommendation for an auto is one of the Benelli versions, although I'm sure there are other very comparable ones out there. My Monifeltro has been with me for 22 years and I'll not likely ever get another waterfowl gun.