RE: What is a good duck gun?
Remington 870 Synthetic Magnum Super Express (I think that is the appropriate string of buzzwords) in 12 gauge. Synthetic because duck guns get exposed to water -- pond water, lake water, rain, snow, falling in the slough stumbling around in the mud while wearing chest waders. This is a pump shotgun and hence very reliable, more robust in the presence of dirt and grit, maybe introduced by falling into the slough while stumbling around in the mud while wearing chest waders or getting some cat tail junk blown into the action. This is an inexpensive shotgun, costing around $300. There are other inexpensive shotguns that will also do the job, but I don't have personal knowledge of them.
If you get serious about duck hunting you can buy a nicer, higher quality duck gun later. But this will get you started without breaking the bank. I use the Remington 870 described above, but I am a novice and just getting started.