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I went on sea princess feb/mar 08 it costs 58 US dollars = £29 english pounds for unlimited soft drinks you can get them from any bar. I also recieved a Diet Coke thermal container which I got filled up when going off the boat. It was worth every penny.
Hey, on the Princess no your room key/card is used for all payment onboard the ship, it's know as a cashless system, you then settle the bill at the end of the cruise, (but be sure to keep an eye on it, it is easy to get cariied away when it doesn't feel like your paying for it.
Princess have a soda card system for a one off payment - not sure how much but you get a refillable flask (I am sure it was around $25 in 2003, haven't bothered with it since), if you do go for it make sure you have it without ice because that is usually just about all you end up with.
When you join a Princess ship you will be able to pay $30 - $40 (charged to your on board account) this will entitle you to unlimited soft drinks coke,sprite etc for this one off payment. This can work out a good buy if cruising with children or to a hot climate.
On the Princes cruises you will be asked as soon as you go top side if you want to buy a drink container with the Princess logo on it and for about $25 you get a sticker put on it so you can get unlimited soft drinks from any bar
hope that helps.