Any advice is greatly appreciated.

That really sucks. What you may try to do is maybe call the cruise line and explain you found something in your stuff that belongs to "Dave" assuming thats his first name, and that you would want to return it to him. That will narrow the list . Though I am not sure how legal that is , you can buy something to make it look real good luck!

Wow! I had this exact same problem two years ago!  I actually called someone else that we hung out with that week and got his number from him.  So if you were with other people that week, and you got their contact information (and hopefully he gave contact info to them) you could get it from them.  I really hope you find him... I can only imagine how devastated I would be if I hadn't had the chance to get in touch with him.  Good luck!