Shotguns:
If I could have only one gun, I'd choose a 12-gauge shotgun. Not only is it probably the best choice for a home/onboard defense gun, it can be used to put food on the table, or even used to launch aerial distress signals, and there are a wide variety of loads available. Remington, like other companies, has a variety of models that are designed for wet weather and some will withstand harsh corrosive environments. The John Browning designed Ithaca Model 37® is about as basic and reliable as you can get, and a shotgun that has been in production since 1937. If storage is an issue, there are models such as the Mossberg JIC™ that can be safely stowed when storage space is limited to a damp, humid environment.
Mossberg JIC™ II
Mossberg JIC™