I don't post alot, but I thought I would weigh in, as a new mom. DH and I were the couple that couldn't imagine taking an infant on a cruise After we found out we were expecting we started planning our yearly cruise as usual with the full intention of leaving our 4 month old with my parents for the week. After Parker was born and our life adjusted, we couldn't imagine leaving him behind. He wasn't going to be old enough for a cruise with us, so we have postponed and are taking him in November, when he will be 14 months old. DH and I evaluated the activities we like to do on cruises (early seating dinner, my afternoon naps in the room and desire to walk around or read a book) and decided that Parker would fit right in.
Parker is the exception to many rules though. He is a very happy baby and always has been. Has slept through the night since 6 weeks, even if he awakes at night he does not scream. I realize some babies might, but I anticipate him possibly being hungry and have a bottle ready to mix for him. He fusses quietly if he wakes, I had him a bottle and he's right back to sleep. DH and I take him to movie nights with friends and out to restaurants regularly (at least 3 times a week) and the only "scene" he caused was when he got choked and scared. DH grabbed him up and took him out. I know that Parker will love cruising based on his easy going attitude.
What I'm saying here, is I would NOT book until after the baby is born. That way you know he/she will meet the age requirements and you can get a general idea of the personality. Some babies are very needy, some are colicky. I would not want to hear a colicky baby all night. There is a board here for family cruises that has tons of information on what you would need,etc.