Usually I take cash onboard with which to gamble with. But, I have a question: On HAL, can cash for gambling be acquired against the credit card 'blocking' that is used to purchase other items?
I would say this is possible on most Cruise Lines, but like Kuki said, all of the Carnival Corporation lines do charge a service fee, and I personally feel that you should only take cash into a casino as that limits how much you spend, especially if you avoid spending anything more than you brought. This is really how you can win over the short term.