Its the electrolysis caused by the salt more than the moisture in the ocean. The same ships hardly rust in freshwater. High humidity was a problem for celluloid film and most would have been using tri X which was developed in the mid 50's with a celluliod base I beleive. The insects where a problem for the paper. I have a whole scrap book full of photos from my granfather from WWII from the South Pacific. They took ships over he got stuck for 4 years with his undeveloped film and developed it when he got back. Much of it was lost.

Used to have my own dark room.
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Liberalism is a mental illness!