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Prepaid credit cards don't all work the same way so you need to ensure OV accept it before you go. If it has Switch/maestro, Visa Electron or Solo on the reverse then I don't think they will. Before you get one of these cards you should consider that there is a charge per transaction for using them. Also what currency are you going to load onto it? OV operate in sterling. Unlike many other cruise lines OV also accept visa debit cards (see their brochure - At Sea and Ashore section towards the back), so if you have one of those why bother to pay to purchase and load up a prepaid credit card and incur all the charges that they impose. If you are unsure I recommend you phone them.
Yes if you have registered your card it will automatically pay your balanc.