When it comes to resisting corrosion in the harsh conditions encountered around salt water, passivated 316 Stainless Steel is the undisputed champ. Its high Nickel content together with a small percentage of Molybdenum make it exceptionally resistive to corrosive salts. This is really the very best choice for all construction exposed to salt spray.
Remember that 316 stainless steel is more prone to breakage than the hardened steel screw you are used to using. Also, note the non-standard head diameter (0.344). Drive carefully! Made in Taiwan.