I have done many cruises solo. Started when I was single, but even now, if hubby can't go and a deal comes up, I'm booked. I actually enjoy it. Pure self-indulgence. I am lucky to have a husband whom I love traveling with, but a week of pure spontaneity is also very refreshing. WHen solo, I always go to the maitre d' as soon as I board and ask to be seated with other solo passengers. And, I have found that morning coffee on deck offers lots of opportunities to meet people. The more crowded it is, the more acceptable it is to just go up to a table and ask if I can join them. Same is true in almost any bar, 'tho I am a very light drinker. Ships are very friendly, as a rule.
I just would not apply ANY of the above if I ended up on a swingers cruise!