Back in the good old days of the 1950s, most ocean liners had a few single cabins available for their single customers. The builders of those ships realized that, yes, there actually were single people in the population! 

Since then, however, all cruise ships have standardized on double cabins, and payment is virtually always based on double occupancy. Persons occupying a double cabin alone are generally forced to pay a single supplement, which is usually 50 to 100 percent of the...