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.
$10/day is added to your account on Carnival, adjustable(but I highly recommend you do not go less than this) except for drinks and in the casino. Also, Carnival allows you to prepay your gratuities, so I would consider this option as well.
Tipping on board a cruise ship is very different. They tend to like it to be very open, as it can motivate other passengers to follow suit. That's if you are doing it in person, rather than auto tips.