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What parts of your firearm are you finding rust spots on? Are you holsters retaining the salt water? Or something else you can change to reduce the chance of rust?
One of our moderators did a practical test of different fluid protectants specifically against salt water. I'm too tired to find the thread right now, but maybe he can stop by and give you the inside scoop.
Any coat that is not metal (duracoat) would be more effective in stopping rust.