I've been on 4 cruises, 2 with friends, and 2 by myself. The 2 by myself were on Carnival, and I would never do that again. Carnival is to cruising, as Circus Circus is to Las Vegas. Yes, you're on a ship, but it feels cheap and not that relaxing compared to all the other cruise lines out there. To be fair, I went when school was out, so it was 99% families (married couples with children), then me. Seriously, I asked to sit at a table with singles, and I was the only single person on the whole ship!
This time, I am giving a "gay group" cruise a try. It is a normal cruise, but a gay travel agency is booking a bunch of rooms and coordinating events. With an interior room, the price by yourself is not that expensive. I booked an 11-day cruise on Celebrity, with Pied Piper Travel (who does a gay croup cruise almost every week). Good luck!