Cruise lines expect to get 25% of their revenue from "extras" like alcohol, shore excursions, etc, so they dont like singles. They want two people (or more) per cabin to buy booze, trips, bingo tickets... So most charge per cabin -- the fare you see is just half the per cabin fare, and there's a 100% single surcharge. There are some exceptions, and they sometimes run specials. My mother cruised Holland America alone - the single surcharge was something like 50%. Some of the higher end lines and boutique lines also have lower suplements and some have cabins specifically for singles. Linblad comes to mind -- Nat'l geographic's line -- I know on the ship that does the Galapagos there is a single cabin or two.

Also, if you're .. .outgoing... most of the lines will hook you up with a roommate. Some I think even say that if you agree to a roomie, and they dont find one, THEN you dont have to pay the single supplement.