When packing for a cruise, my motto is: Take out half the clothes you packed, and replace it with twice the cash!
I always seem to not need most of the clothes I pack (I spend a lot of time in a bathing suit!) and I always see stuff I want to buy in the ports.
Another important tip if you're in an inside cabin, bring a plug in nightlight.
Those cabins are pretty dark, and when your wakeup call comes at 6 am and you can't find the phone, it can be pretty comedic!