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It depends on the cruise line. Every ship has a single supplement, e.g. P&O from 40% to 80%, but it is a double cabin. I think some of the smaller ships still have single cabins, but you still pay extra. Cunard appear to be 100%, certainly on the QM2. Very unfair, but it is the same with hotel rooms. You need to check every cruise with every ship. Such as the baltic and Xmas cruises seem to attract the highest supplement, and the Caribbean and Canaries the lowest.