I solo-d on the last cruise and thought I would be MIS-ER-AB-LE. I now don't think I would EVER want to go on one of these things as a couple. I actually went with a couple of friends (a couple) and "lost" them for the first two days. That turned out to be the best thing that could happen to me. I met so many people whilst wandering around looking for them.
By day 3, we had a growing "crew". We closed the club every night and partied on the deck until almost sunrise. Trust me...you won't be alone.
My advice - just talk to people...on the elevators, at dinner, at the pool. You'll be surprised. Most people will at least chat for a minute. I actually ended up becoming good friends with people that I just randomly met on the elevators. Once people start meeting each other, they start networking very quickly and the next thing you know, your 'new' friends will be introducing you to their 'new' friends.