We tend to overpack on our trips. We try to bring extra batteries and lots of film for the camera. We bring an underwater disposable camera for snorkeling pictures at castaway key. You do not pay cash for anything on the ship unless you want to settle your shipboard account with cash. That being said you will want to bring some $1.00 bills to tip the servers from room service. We also bring a small supply of medications (cold, antacid, asprin, and diareah). You can get the necessities on the ship but the cast a little more. Soda is free on Deck 9 but you might want to bring a mug and even some soda for your stateroom.
When you pack your clothes you can wear your Tux/suit for both formal and semi formal night so you only really need one. If you rent one through the cruiseline you dont even need to bring a jacket unless you plan to dine at Palo. We enjoy the interaction with servers and the family at Dinner and have never eaten dinner at Palo. You can get by with about 2-3 pairs of dress pants. My wife brings 3-4 dresses and a couple pantsuits with interchangeable tops. Im sure I am forgetting a buch of stuff but I am sure someone else will cover it.