You can’t withdraw the money because the account “settles” the last night of the cruise. However, have you checked with your credit card company about fees? We are in the opposite situation often and my credit card companies actually adjusts to the best exchange rate for the billing cycle. Carnival will place “holds” from the sail and sign account then they back those charges out and push one “actual” charge after the cruise ends. So there’s only one “real” charge from carnival. I do wonder if anyone has used a prepaid (US) card to avoid foreign transaction fees, because that also may be something that you could do.