This is a fantastic thread! I'm making my packing list for my next sailing and this helped me remember a few helpful things that hadn't made it on the list yet!
Here are mine, don't think they've been mentioned:
-Zip ties. This is a good way to know whether or not your bag has been opened without risking putting a lock on it that could get broken
-Duct tape. You never know
-A small dry erase board. Great for leaving notes for each other on those busy days or for a note/greeting for your cabin steward.
-Laminating your baggage tag. For those of you who want to be extra careful.
And the one's that have been said that I'd stress:
-Highlighters, travel mugs, bubble wrap, wrinkle remover (unless you do the good ole fashioned shower steam hehe), an open mind, an appetite and a little connection to the outside world as possible