The Marine Magnum is only offered in 18" cylinder bore barrel, though if you bought a spare barrel for an 870 it would fit. But it wouldn't be legal for hunting unless you made a plug to block the magazine capacity to 2 rounds. A standard plug would be too short because on the extended magazine cap on the gun and the gun will actually take 3 rounds in the mag. Remington does not offer any other barrels in the marine finish. If you wanted another barrel in a longer length for hunting, there are many companies offering a hard chrome refinishing, and you could buy a 870 barrel and have it refinished.
The finish is very durable and is naturally slick so you don't have to use alot of lube. A customer that bought one from me keeps his in boat and he's had it there for 2 years with no rusting, or surface discoloration.
I don't think you could go wrong by purchasing one of these.