We have been in this postion before, and the easyest thing to do is to take Travellers Cheques for the days ashore and pay the amount of cash you wish to spend on board into the pursers office on board. This can be checked every couple of days to see how much you have left, and of course nobody spends the same, we have been on cruises where people have spent less than £25 because they dont drink, and tea and coffee is free.And yet some people drink champagne every day.
It is not that difficult to work out roughly how much your onboard account will come to. Drinks, including soft drinks, beer/wine and not forgetting bottled water etc cost about the same on P&O ships as you would pay in a similar type of pub or restaurant at home. They are not cheap, but they are reasonable compared with some other cruise lines. I would really suggest that you get yourself a credit card, if only for this occasion, and then cancel it afterwards if you wish! It really does take the hassle and worry out of paying your onboard account. You can even pick ones up with a few months interest free credit on them so that you don't have to worry about immediately paying for your cruise spending as soon as you get back home! If you get one that does not charge you a fee or commission for use abroad, such as the Nationwide, it will actually save you quite a bit of money as it means that you can make larger purchases, such as clothes, photographic items, or more expensive souvenirs without having to keep topping up your local currency or digging into your travellers cheques. Happy cruising.