Hell, I don't know of a PUMP or such in stainless. Ruger made a short run of Red Labels in stainless, I've heard. Second best option IMHO is a camo dip finish, very tough against rust. You have to clean the gun, of course, but it has held up well on my Mossberg for 20 years of salt marsh and bay hunting out of boats, kayaks, and on foot. One can have a Spartan/Baikal or old Savage or such dipped. You can also just past wax the gun, seems to work fairly well. I am paste waxing all my field guns now, even the camo 500. Since I'm a hunter, I'd camo dip even a stainless gun to keep the glare down. Stainless would flair ducks left and right, I'm afraid, if the sun was out.
BTW, there is absolutely NOTHING unreliable about my Spartan in the 5 years of my ownership. It's a tough little gun. I refuse to believe that it's the POS everyone tells me it is. Hell, it's a simple break open shotgun, no shell elevators, magazine latches, pump handles, bolts, just a simple break open. Single shots MIGHT be simpler with a hammer. I have an H&R 10 gauge that's pretty simple and didn't cost 5 figures. Why should adding a barrel make it less reliable? Mine works. I have a Sarasqueta I paid 100 bucks for 42 years ago that still shoots fine. The salt marsh and boat rides did a number on its looks, but it still shoots fine. I have an old Iver Johnson break barrel single shot I have no idea HOW old it is, only that it was discontinued in 1947. It's tight as a drum. No, I've not used it much since my youth, though. I haven't used that old Sarasqueta much in 25 years, either, but it was once my do all hunt everything shotgun. I used the snot out of it when I WAS using it.
The Mossy is a solid weapon. Wax it, keep it lubed with corrosion X. It shouldn't age too badly. I've done the say with 500s. If you're worried about rust, the one choice I'd avoid is a Remington Express model, a rust magnet. Most folks I know that hunt the salt marsh with 870 Expresses sand and spray paint the things and touch 'em up occasionally.
