
Metal roofing should always have underlayment beneath it, if that is what you're asking. We'd be happy to discuss your installation in more detail if you wish. Also, for a corrosive environment such as this, I'd sure consider aluminum rather than steel.
I recently purchased a house in the Caribbean which has a asphalt tile on the roof. I want to add a galvanized roof on top of it plus I'm adding insulation to reduce heat transferring into the house. My question is, do I still need to add a moisture barrier?