If Carnival does what RCCL did to me a few years ago, they will allow you to establish a 'cash' account with them. They have a minimum deposit of somewhere in the neighborhood of $250. As soon as your account begins to run low they will notify you that you have to add more to it. I assume if you don't they cut off your S&S 'privileges'.
I put $500 into my account upon boarding my RCCL cruise, and within three days I had a notice tacked to my cabin door saying that my account was low (I think I had gotten to within $100 of a zero balance) and that I needed to come to the purser's desk. Here I thought I had gotten an invitation to dine with the Captain...
Ended up spending about $800 onboard ship, not including gratuities. That was just for pictures, drinks and a few gift purchases. And I don't drink much!