For singles, a nice thing about cruises is that you needn't worry about dining alone, since you'll be seated with other guests (if you don't want to be, seek a ship with alternative dining options). You also needn't worry much about finding people to talk to, since the general atmosphere on nearly all ships is very congenial and allows you to easily find conversation, especially during group activities. And the ship may even host a party to give singles a chance to get to know one another and/or offer social hosts as dance partners.
The downside is that you may have to pay more for the cruise experience than those sharing a room. Since cruise line rates are based on two people per cabin, some lines charge a "single supplement" rate (which sounds like a deal, but it's you who pays the supplement) that ranges from 110% to an outrageous 200% of the per-person, double-occupancy fare. As a single person, you have two choices: Find a line with a reasonable single supplement rate or ask if the line has a cabin-share program, under which the line will pair you with another single so you can get a lower fare. You may not be able to get much information about your roommate before the sailing, although all lines match gender and most also try to match age. Some lines also offer a single guarantee program, which means if they can't find you a roommate, they'll book you in a cabin alone but still honor the shared rate. On some older ships (including the QE2) and a few small ships, there are special cabins designated for single travelers, and in some cases they carry no additional charge. But keep in mind that these cabins, originally designed on the older ships for nannies or maids accompanying passengers, are really, really small, and that they tend to sell out fast.
To increase your chances of meeting other singles, book a cruise through a travel company that specializes in bringing singles together. These companies include Cruiseman (tel. 800/805-0053; www.cruiseman.com) and Discount Travel Club (tel. 800/393-5000; www.singlecruise.com). Such firms coordinate groups of as few as 30 or as many as 300 singles on specific sailings, and typically have a tour coordinator on board to organize mixers and make sure people get a chance to meet. Singles in these groups tend to be in their 30s to 50s (of course, some may be younger or older).