Sounds to me like there's no salt water proof shotgun out there. Given that perhaps buy the cheapest thing that will work decently, try to slather it down with something to slow the corrosion and clean it each time you get back. And in the end consider it as a "consumable" so when it rusts past the point of no return in two or three seasons you just write it off and get something else.
This whole idea also seems to indicated that a used and cheapie SxS that is greased up on the outside and that you use a pair of barrel plugs and grease the hinge joint would be a great idea. When you load just yank on the big red "Remove Before Flight" banner attached to the barrel corks and load up. And because the action on most SxS's is mostly sealed from the outside it should be decently easy to keep the action lubed and rust free. And by using the barrel muzzle plugs and greasing the chamber ends the barrels should not see salt air until you break it open and load it. This would restrict contact with the salt air on the barrel bores to a minimum. With some care the gun may well last for quite a few seasons. And when it does go bad then you're only into it for a couple of hundred for a cheapie.
