No matter where you go in the world, it's a hard fact that traveling solo isn't as cost effective as traveling as a couple or in a group. You can't split the cost of hotel rooms with anyone, and a lot of the entertainment out there is geared toward couples.
The preference is for couples in the cruise industry too: the rates they give you are based on the assumption that two people will occupy your cabin. If it's just you, expect to pay one and a half to twice as much.
So naturally you might assume cruises just aren't good vacation options for singles.
But actually, there are plenty of ways you can cruise solo in an affordable way. For instance, some cruise lines will match you up with other singles (same sex) to have as a cabin mate.
Since you probably won't spend much time in your cabin anyway on a cruise, it doesn't matter too much about who the other person is, but who knows--you may hit it off and then you'll have someone to dine and take shore excursions with.
As a perk, if the cruise line can't find a single roommate for you, you'll get the cabin at the same price (no paying double for that unoccupied bunk) and have it all to yourself. (Make sure to check if your cruise line has a program like this before you book. Some do and some don't.)
If you've already booked a cruise, or the cruise line you want to travel with doesn't offer a singles match program, head over to the Cruise Mates message board and check out the "Seeking Cruise Companion" forum. You can post your needs and browse posts from others who are looking to save money by finding a cabin mate.
Some cruise lines specialize in cruises just for singles. Others offer activities for singles about mainstream cruises, to help you meet others who are traveling solo.
So don't assume that all the activities are meant for families and couples. There are lots of cruises out there where the solo traveler can have a good time.