The combination of salt water, sun and wind make the marine environment one of the toughest places to combat rust. Any steel surface that becomes exposed because the protective coating is chipped or worn away will start to corrode almost immediately. Once rust has started, it takes drastic measures to curb its spread.
Because of friction and the potential for even slight movement of parts relative to each other to wear away or crack coatings, moving parts and fasteners are usually most susceptible to rust. As soon as the metal is exposed to oxygen, it starts to rust.
So be on the lookout for the start of rust wherever metal moves against metal, even just seams where two plates connect. Uneven expansion and contraction is enough to rip protective coatings and allow air and salt water to enter. Polytetrafluoroethylene (PTFE) polymers and dry-film lubricants are recommended in these places.
Even surfaces free of seams and movements can be at risk if there are any holes in the coating. Thick coatings such as epoxy and urethane provide good protection for such surfaces.
Radiation and temperatures could also pose risks, as organic-based coatings tend to become more porous at higher temperatures, opening the door (microscopically speaking!) for oxygen to penetrate and reach the metal substrate.