I wasin Cape May Nj for four years and had a small studio in my garage. I was 7 blocks from the ocean. Its tough on everything there. Homes, cars kept outside, etc. Had heat/air in the studio so mostly kept the window closed and didn't notice any equipment problems. Sometimes just ran the air unit on "dry" setting to get out the humidity.
Just did an outside gig here in the Lowcountry ( translates to super humid ) of South Carolina and the evening mist was so bad I kept wiping my keyboard off and covered it during our one break.
The BK9 is a plastic case but I worry more about what happens inside. I guess all you can do is be as careful as you can and keep all you gear in cases or bags when not in use.