You want pump, semi auto, or 2 barrels?
If you want a pump, you can't go wrong with the 870 and BPS recommendations. A used 20 gauge 870 Wingmaster would be nice and save you some cash over a new gun. If you don't care about cosmetics and are getting an 870 Express (which would be my choice in a duck gun that will get muddy, sandy, salty and wet on a regular basis), you'll still get a bomb proof gun. You won't save much buying an Express used, though.
A BPS will be a little more money, and a little nicer finished. If you're looking at those two guns, you should also be looking at the Mossberg 500, which is very similar in quality and price.