I read all of the postings here and noticed that no one mentioned an amount. Depending on if you are on a budget on this cruise (I remember taking cruises while I was in college).. you might want to know that the 3 night cruises require that you have at least $200 in your on board cash account.
And if you really don't want to get a credit card, and I know people who are against it... you can always go to the pursers desk the night before and pay off your account- you just can't use your sea pass card anymore if you don't want to have to stand in line in the morning to pay off whatever else you spend. It is a pain but there are obviously a lot of people who choose this method... or the lines would be alot shorter.
Have an awesome time! Happy Sailings!