I think the real thieves and vandals are the waves, and the boucing and banging of the boat. Secure your stuff, and use you imagination to realize when something could be lost or damaged by bouncing, banging and even a the occasional large wave. On our last trip a rinse tank broke free and washed overboard on an overnight crossing. Now admittedly one should have left stuff in the tank, it could have bounced out or even stayed in the tank, but banged around enough to get broken. In this case the whole darn thing ripped loose and went overboard. One fellow lost his camera, another diver lost his mask and computer.
Your gear will usually be in a basket under your place on the bench. The basket could spill over in rough seas, or the things in it could bang together, crack the case on your flashlight, maybe even bring your weight belt and computer into unfortunately close contact. I usually secure my mask and computer well away from my weights and take my camera to my room, never leaving anywhere it could fall - think in a drawer not on a table. I also try to make sure my 2nd stages aren’t free to bounce around and bang against the tank/a wall/etc.
Do label your stuff though. I made a dive with the wrong mask once - I and the other diver should have been as blind as bats with each others masks, but by some miracle we both had equally strong corrective lenses. Nice that our dives weren’t ruined, but it’s a mistake worth avoiding.
Having a lot of black gear, I love my white paint pen (sold in automotive stores as a tire marker). I keep one in my toiletries bag so it will never be left at home. Try to label the correct part(s) of your fins so the deck guys can ID your fins at a glance.