Last January, my intended cruise partner had to work at the last minute and cancelled only 4 weeks before our sail date. I connected with a great guy to share the expenses with, not expecting anything more than a way to save some money. To my surprise, we really got along and spent quite a bit of time hanging out... strictly as friends. We both agreed that we would cruise together again if the opportunity ever presented itself. Some people we met on the cruise remarked how "brave" we were to try this, but my answer was, "it's a cruise! What's he going to do... rape me and swim home?"
... take a chance! You might really enjoy yourself!
I think the key is to exchange phone numbers and at least talk on the phone a few times to make sure you feel comfortable with each other. It was a great experience, though, and i'm currently advertising for another cruise mate for another cruise.