Remington's new Marine Magnum 870 Tac-14 Shotgun
Remington Arms is now shipping a hybrid marinized version of their Model 870 TAC-14 shotgun. It's priced at $870, which is about double the price of the standard model.
With some of the high-quality lubricants available on the market today that are capable of keeping regular 'blue genre' firearms in pristine condition, the operating conditions would have to be pretty rough in order to justify the added expense—but I'm sure that there are cases that fall into that category.
/fl