You will need to give them a credit card number and at the end of the cruise they will charge that account the amount you've used during the week. YOu can check your account when you want to and the last evening, they will give you your total detailed bill. Good idea to keep track of all your purchases so it won't be a huge suprise.
I couldn't find where it said the exact amount but this is what I copied from the carnival website
"Carnival accepts American Express, Visa, MasterCard, Discover Card and cash for deposit on your account. A deposit (ranging from $100 to $350 depending on cruise duration) will be required for cash accounts. "
When I went on my honeymoon (October 2004) we went on a 7 day cruise. I believe they only put a $200 "hold" on our card for the sail and sign. Nothing was charged on it until we told them to put it on the card. You can pay cash for it if you would like. They just put a hold on it, so we couldn't run out and not pay for it.