Many people think that use AISI 304 or AISI 316 could solve all the screws rust problem but it’s not true.
For sea water applications, standard stainless steel fasteners, don’t guaranty durable fixing systems for all your equipment because rust could be affected also these stainless steel families.
Chloride ions can cause localized corrosive attack (pitting and crevice corrosion) of susceptible stainless steels. In a sea water environment it must be made clear as to whether the stainless steel is submerged, the seashore such that it can be attacked by the chloride present in seawater by splashing or carried by onshore breeze.
When the stainless steel will be submerged, a pitting resistance equivalent number (PREN) greater than 40 is typically specified as the minimum for resistance to seawater. Stainless steels, such as super austenitic stainless steels (for example UNS S31254 or 904L), or super duplex stainless steels (for example UNS S32760 or S32750) meet this requirement.
For sure, in order to evoid the rust problem the best and cheaper way you have is adopt Duplex Stainless Steel screws the right compromise between price and performace.
