My sister, who has travelled quite a bit, and who also owns a limo company and drives, has given me many many "tips" on tipping. One being the unobtrusiveness of the exchange. Holding a bill in your palm, and extending your hand, palm down, often to the side of your body, or even someone behind, is a universal action, and easily managed at airport, hotel, restaurant, and I would assume cruise ship also. Personally, I would not want to draw attention to myself by tipping in a manner that become, shall we say "public". Just my opinion.