I am really looking for a defense gun with waterfowl capabilities.
In a 12 ga, pump action. The only thing I'd look at to cover BOTH of those applications VERY well, personally is a Mossberg 835. Home defense barrels available, tough as nails and with the ability to shoot 3 & 1/2" mag shells; it can handle anything you could think of to through at a shotgun....and then some. Easy to find also, and cheap. I do like the remingtons, but 90% + of the 870s I come across are 3" guns.
I own two of them, I plan on picking up a third this year. A Parked. or blued model with wood stocks I can find one. Do I have others that are fancier for show, that I grab out of the safe before I'll reach for the mossy for specific uses? Yes... I've got down right pretty auto loaders for upland birds and skeet. But the 835s are my go to workhorses.