The "best" protective coating for carbon steel depends on the environment the steel will be exposed to and whether the surface is new, old, or has been previously coated. Will the structure or item to be coated be inside or outdoors, subject to heating, cooling, wind, weathering, periodic or seasonal temperature changes, dry or humid conditions, etc. Just like doing painting at home, surface preparation is extremely important. Not all surfaces are suitable and/or available for all types of surface preparation techniques. From your question, it sounds like the surface of this carbon steel will be frequently wetted by freshwater from condensation, splash, or spray. According to Schweitzer in his book  Corrosion-Resistant Linings and Coatings, the best systems for this environment make use of inorganic zinc as a primer since it provides the longest term of protection. Recommended systems for this environment include inorganic zinc/epoxy primer/polyurethane finish, inorganic zinc/acrylic finish, aluminum epoxy mastic/acrylic finish, and epoxy mastic.