The Mossberg 835 3.5" pumps are designed for harsh environments.
Unlike the Remington 870/1100/1187, the internal shell latch can be removed easily. This allows flushing salt water out. The Remington latches are staked in place.
Unlike the Mossberg 500, the 835 mag tube is a copy of the milspec 590 tube. This was designed so soldiers could thoroughly clean out the inside of the mag tube.
The Mossberg receiver is aluminium, so it resists corrosion better.
Though available on guns from other makers, the Mossberg can also be had in a camo dipped version, providing extra protection for the metal.
I don't have a 835, but I do have a 590, so I'm familiar with the above features.
I've posted a link to the 835 waterfowl page. While the default gun is blued with birch or beech stocks, there are synthetic versions available via the pull down menu.
For a well made shotgun the price is quite affordable.
You may not need 3.5" shells, but the option is there with the 835.