Yeah, you do need to watch out for salt air. Silica gel is fine for moisture (though you need to chase the moisture out of them at regular intervals), but I'm not sure what you should do about the salt air. But if you're not right by the sea, it's probably much less of a factor. I'm about 100m from the sea front, blocked by another row of houses but still the proximity doesn't help. I have picked up silver and gold from under the sea with little effect on the piece ( though they were never going to be F/GF coins) its the copper and other "rust-able" items that i was thinking about?