Anything with a bar code will work, it needs the exact same card to open as it was closed with.
CruiseDad, Just a minor correction so we don't confuse anyone. It is any card with a magnetic strip.
I have never had anything taken either. I had misplaced a gold ring once. It must have fallen under the bed or something. I didn't mention it to the steward. I came back from dinner and it was sitting on the bathroom vanity on a tissue.
Some ports I don't worry as much as others about leaving things while snorkeling. It is always good to be cautious in places you are not familiar with. When on St. John and snorkeling at Trunk Bay, I just leave everything beside the lifeguard stand. I wouldn't do that on any other beach though.