Disney Cruises for people without kids.....
I would never go on a Disney Cruise without brining children. I like the BYOB but I would rather pay for my booze than deal with tons of other people's kids all the time and have to watch Mickey Mouse shows the whole time. Every cruise has children, but I think you are asking for trouble being a 20-something on a cruise line that caters specifically to children. I'm sure Disney cruises are fun but I just really don't like dealing with tons of kids on a vacation that is supposed to be relaxing.
NCL has three ships that have single studio rooms and a special lounge for people who book those rooms. I think a cruise like that would be a good way to go if you are looking to meet other singles.