Check out options from tour companies like G Adventures and Intrepid Travel that allow you to pair up with a same-sex roommate during your trip as a way to get around the pesky single supplement. Grand Circle Travel offers the same service for travelers ages 50 and up while Contiki offers a similar plan for 18-35 year olds.
While you'd normally have to worry about paying double (since cruise cabins are almost always based on double occupancy), Norwegian Cruise Lines has another, more affordable option. On the Pride of America ship (based in Hawaii), Norwegian Epic, Norwegian Breakaway, and the soon-to-be-christened Norwegian Gateway (coming in February 2014), you'll get your own room and the chance to mingle with fellow solo cruisers in the Studio Lounge, all for less than the cost of your usual solo cruise ticket. From $1,099 per cabin; check NCL.com for pricing details based on your actual cruise dates.