I have an air ride pin box and I like to try to keep the sun off the air bag to avoid weathering of the rubber. The way I do it is use a black plastic trashbag, put it over the whole assembly, bunch up all the slack in the bag up underneath to keep it from looking unsightly and making noise in the wind, then wrap with black duct tape. Poke a small hole at the lowest point to allow water out if it gets in there. Cheap, simple, disposable. Works for me :)
If you are worried, coat the exposed metal with grease or (preferably)drylube which can be wiped off once you leave the beach.
Our 5th wheel is stored 6 miles from the ocean and we stay 3 months each year less than 300 yards from the ocean. I've never even worried about the pin area or done anything to protect it or the hitch in the back of the truck.
Our 5th wheel is 20 years old and I don't see any signs of the salt air beating up my pin or pinbox.