BINGO!
only stuff inside a "protected" area get stolen! Leaving stuff in the open dosen't make it look less valuable...no, but what it does do is tell your visitors: "Look, we trust you to act like a good visitor and not a crook." That is a big part of the equation. Not the only part though. It also helps that wehave only vertical ladders (can't really climb up with a torpedo under your arm) and a general look of an operational ship helps too!
IF your boat looks like its ready for the scrap yard, the subliminal message to certain visitors will be "Get your souvenir NOW BEFORE THIS BOAT DISSAPPEARS FOREVER!!! We lost lots of stuff before the restoration program really took hold. Then it trailed off to almost nothing (almost).
When you think you've gotten to that point test your waters -- Start by putting out non-historial replicas or nearly-correct stuff. When that doesn't walk, and you feel emboldened, then move to the real deal.
Again, it isn't perfect. We lost a USN spoon the first month of our season. But to my knowledge, it is the first such loss in several years. In the eight years we have had a mess table set with knives, spoons, forks, plates, handless cups, and soup bowls, we have lost maybe two or three implements (I buy replacements in thrift stores (YES THRIFT STORES SHOULD BE CHECKED FOR USN flatware) and ebay.
Good luck!