Thinking about picking up a Winchester SXP Marine Defender. I can get into one under $400 but I see they are made in Turkey these days. From what I can see on reviews they are OK what does the hive think. This would see some use on a boat.

I would chose the Mossberg 500 Mariner instead. Made in USA and has a ton of companies making accessories.

Ive never used one but I'll say this: Where I live is serious "boat country", everybody has a boat and everybody keeps shotguns on the boats.

I'm not saying Nobody here has the Winchester but I'll tell you that out of the dozens if not hundreds of boats I've been on or people I talk to, nobody has that Winchester.

The Remington and Mossberg marine guns are what everybody has and its about an even split between the 2 I have both and for me, its 6 of one, 1/2 dozen of the other.

No help on the actual Winchester involved. But I'd guess based on the description of the finish its probably not great. I've kept a shotgun on a number of boats for decades now. In truly bad storage conditions. The best has been the Rem 870 marine. Second best is the older win1300 marine. I had a Mossberg and it didn't do well, but that was some time ago.
Summary this would not be my first choice.

A nitride or stainless barrel AR15 makes a good boat gun as well.