You can use either debit card or cash.
If you use a debit card just remember there will be a daily "reserve" put on the card that will somewhere around $250. They don't actually take the money but they lock that amount in. It is the same if you use a Credit Card.
If you do not have a credit card then I would recommend setting up a cash account. You will need about $250 and when that runs down Customer Service will let you know and you can add more to it. If you have any left at the end of the cruise it will be refunded, by check, to you.