I would recommend a Benelli Nova. Myself and a few friends have them and they are very nice guns. They are extremely weather resistant, and cheaper than the 870 super mag. They cost around $350 for cammo. They also have a lot of good features and the forend rattle everyone complains about can be greatly reduced by tightening up the two allen screws on the forend.

The Nova is the only pump shotgun I would own for waterfowl.