[Fern]

Here's the info from the Carnival site:
Selecting the payment method



When setting up the on board account you can select either credit account or cash account (U.S. currency or travel checks) as the payment method. Cash accounts require a minimum deposit. The recommended deposit guidelines are as follows:

1. • 2-4 day cruises - $100 deposit per person
     <5-8 day cruises - $200 deposit per person
   • 9 + day cruises - $350 deposit per person
2. Hope this is helpfull.