I'm not going to second guess what anyone should or should not do & I'm not going to judge. I'm just going to provide a little bit of information on some hardware that is available.
Most “fireproof” safes are made of sheet metal with refractory cement between the layers. Those will absorb moisture & swell in humid environments. Eventually, they swell so much that they either don’t open or don’t close anymore. About that time, they usually start to rust noticeably.
Cheap “safes” that are not fire rated, usually are made of slightly heavier sheet metal. They usually have electronic locks that can be thwarted by people who know what they are doing. I’m not going to post the methods. These “safes” usually also have a key back up for when the batteries in the electronic lock go dead. The batteries can also fall out of their holders when operating in heavy seas.
Slightly better (non-fire rated) safes, can be made of plate steel or proprietary composites. These may have dial locks & or key locks. They may also have exotic electronic locks. Most electronic locks are susceptible to dead battery syndrome. A few are self charging when you spin the dial. All electronic locks are susceptible to death by corrosion at sea. Dial locks should be cleaned & lubricated by a competent individual every 6-months in a marine environment that does not involve direct water contact. Key locks in the same environment should be cleaned & lubricated annually.
Good safes have ratings that start with the letters TL & are probably worth more than the “valuables” that you would want to keep on a boat that does not have a full time professional captain & crew. Really good safes have ratings that start with TXTR & are beyond the scope of this discussion.
Anything marked RSC will probably meet legal requirements for items that you are required to lock up, but is not something that I consider a real safe. Class B burglary ratings are a little better than the average RSC. Class C is a little better than that. You are going to spend several hundred dollars for a small Class C box.
