Where we live, it's hot and humid and a lot of salt breeze. It really does depend on all these details and then actual nearness and exposure. Something that is 30 meters from the water and exposed lasts 1/4 of the time as something 100 meters from the sea with a wall between. Keep your gear as far out of the sea breeze as possible.
I found that keeping dust down also really helps. In addition, I have sealed boxes with gun safe dryers in them for gear that is stored. The dryers have silica beads and you plug 'em in to dry 'em out, then into in the sealed bin with the gear. Rinse and repeat monthly.
For my computers, router etc, stuff that can be kept on, I keep as much as possible inside a cabinet with limited air flow (not sealed) and let the heat they generate drive the moisture off. Really helps. Probably ten degrees hotter inside and I'm convinced that the heat damages the gear far less than the moisture that is driven off.
Stuff that is in the open, keep the air moving so that dampness does not accumulate. Fans.
Be prepared to replace as needed.
I would never cover anything (except the a shovel perhaps) with WD40. WD40 will cause dust and debris to stick to whatever its coating. That and WD40 dries out leaving gummy residue behind. – Keltari – 2015-09-19T22:32:15.393
WD-40, or Stoddard fluid, ranks at the bottom for rust protection. However as a penetrant, it is good for getting between electrical connections and creating resistance. If you want a good rust protectant with a great history, reputation and the scientific studies to back it up, use Fluid Film. Just don't use it on electronics either. – kmarsh – 2009-09-11T12:34:53.653