Ellen, when you board just inform them that your are going to use cash as your deposit for your sail & sign card. You will need to activate your account at the Pursers desk immediately after you board or your cards will not work. Most people place a minimum of $200 but I would suggest a higher amount unless you plane on really watching your charges and placing more down when you get close to the amount of deposit. This is why so many use a credit card, it is just easier and you can still pay it off immedately when you get home if you have one.
Jim