I do not speak from personal experience in a salt water environemnt (I am an inland trailer sailor) but WD-40, believe it or not, was originally designed as a metal dressing/protector for the old Atlas missle. It has worked well for me the few times I've used it as a metal protector. Needs to be re-applied. Note WD-40 is NOT a lubricant. It appears to work as a lube when first sprayed on something but it turns gummy. Read the can.
WD-40 is an interesting company in San Diego. The product was developed in the 50's if I recall under subcontract to Convair. They always have been and still are a single product company (very unusual). Chemical to make the product come in freight cars delivered right to their plant (it is possible they own the cars). Imagine...an ~60 year old company with a still successfully marketing the single product they invented and started that may years ago. Got milk? Got WD-40?