- developed thoughts and edited-
Stainless shotgun barrels aren't the same as stainless rifle barrels. As far as I can tell, the thin walls of a wingshooting scattergun aren't compatible with stainless metallurgy. Rifle and pistol barrels are thick walled, so it's compatible (I'm not a steel guy, but thin steel needs an "elasticity" to handle pressures, and my assumptions have not been subject to rigorous fact-checking).
The purpose-built defensive shotguns such as the linked marine magnum, I personally wouldn't try to hit a duck with one. Short barrels are hard to swing (yes you _do_ need to swing on rising ducks) and the stocks are designed for a cheek weld (with a low comb you'll need to fully cover a rising target with the barrel to hit) I've shot rounds of skeet with defense setups. They weren't very memorable.
If you're dead set on stainless in one form or another, I'd check if the marine magnums can accept a standard blued barrel and a field buttstock.
On the other hand, if you want to hit ducks, keep the elk gun cased until you need it, nothing to be gained by swapping barrels in the field.
I suggest a field grade 3" 500/870 with a plastic high comb (hunting, not clay or defensive) stock and a 28 or 30 inch threaded barrel. Or Ben/Charby's Benelli. The most important options in your situation are a warm water hose attachment and a case of CLP. It may rust, it may go overboard, you might need to jam seaweed in the magazine because you forgot the plug, it might get bent from using it as a push pole (unloaded of course!)
Sometimes a tool is just a tool, and we spend a lot of time chasing our tail when we could be hunting.