My son was 11 mos old the first time he cruised, and we will be taking our then 5 mos old daughter over the holidays. What do you want to know? I will say you will need to bring plenty of diapering supplies, infant medicines, sunscreen, hat, cover up, feeding supplies (bottles, dish soap, formula if using it -I nurse so I didnt need to deal with formula, and our daugther will just be starting cereal/solids so I will bring a box of that, and puree some fruit and veggies if shes eating that then), etc. Ships dont carry infant supplies and you will waste time and money looking for stuff in ports. Depending on your child, you might still be able to do tour type excursions (not active doing type stuff-more like walks, bus rides). We never bothered with our son. Instead we walked/shopped in port, then headed to a beach for a little while. Keep in mind the sun will be really strong and hot for such a little one, and they dont recommend sunscreen for under 6 monthers.
You will also want a stroller with a good sunshade. The smaller the better as hallways, elevators tenders, gangways can be jammed with people. You dont need the huge travel system, You wont have a lot of room to stow it in your cabin and it will be a burden. For example, I have a Maclaren Quest stroller. It weighs around 13 lbs, can support up to 55 lbs, folds, has a decent sunshade that can be manuvered and is a breeze to push. A small diaper bag for short trips is all I need. I leave the car seat at home since most cabs and transfers dont have seatbelts. I also bring a sling to carry my baby around for short walks. Much easier than a stroller.
In all honesty, its much easier to travel when they are younger. We made sure we all got naps in the afternoon, and dinner was a breeze. We went to shows and our son usually fell asleep (another good time for a stroller). Just be sure to wash hands a lot and just before you eat (like purellas soon as you sit at your table espeically afte handling the serving utensils at the buffet after hundreds of others have too). Also bring a jacket, blanket or sweater to cover baby up in the cool rooms. Be respectful of others around you. If your child gets fussy, leave the situation.. Our waiters never had a problem packing up my meal and sending it to my cabin (only happened once really and my son was sick with a bad ear infection). Plus there are other dining options on board. If you want more tips, search the boards, especially the gripes about small kids and babies. Dont let it scare you though. Some people cant be pleased.