I have cruised with my twins when they were 9-months old and then again when they were 18-months old. Here's my advice:
1) Keep her as close to her regular eating/sleeping schedule as possible. My twins would nap in their stroller or on a lounge chair in the afternoons and this helped to ensure we avoided the "crankies" later on.
2) Vary your activity during the day so that she doesn't get bored just doing one thing.
I found simple walks around the ship to be a great diversion. The kids really seemed to enjoy just going to different places and watching all the people. The glass elevator was one of their favorite places and the gift shop was also always a big hit (especially since they usually came out with a new stuffed animal or toy). I would also often take them up to the tennis court so that they could run (or crawl) off some energy without disturbing anyone.
3) Bring a bag of small toys that can easily be carried around the ship with you. I packed a small duffel bag with a combination of my sons' favorites and new things they had never seen before. This proved to be a godsend during long dinners in the main dining room, as well as long afternoons on deck or other times when I just needed to relax and have some "quiet time".
4) You don't mention how old your baby is, but if old enough, you might want to bring some crackers or other small snack to keep her happy while waiting for the food to arrive at dinner.
5) If your baby is old enough, bring snacks from the ship when you go into port, especially if you're sailing to someplace where you're not comfortable allowing her to eat the local food. This will hold her over until you can get back to the ship to eat. Bringing food off the ship is prohibited in many ports, however I found them to be extremely lenient and understanding when it comes to a small child.
6) Don't be afraid to get her involved in the "nightlife" aboard ship. My babies really enjoyed the colorful costumes and singing/dancing of the production shows. Afterwards I would take them "bar hopping" to the various lounges They enjoyed the bright lights and the music. They loved to just sway to the music and clap from their chairs in their own version of "dancing". Some of my best memories (and best photos) are of the twins enjoying the nighttime activities that the ship offers.
7) Bring any prescription or over-the-counter medications that you think your baby might possibly need. Children's medications are hard to get on the ship unless you go to the infirmary, which is very expensive. By the way, my pediatrician told me that babies don't get seasick, so you don't have to worry about that.
8) Don't forget to pack any of your babies nighttime comfort items. I made it into the car and down the street before realizing that I had forgotten to pack my son's "binkie bear". What a nightmare that would have been!
I found cruising to be the perfect family vacation and can't wait to do it again. Just go in knowing that it will be different from your child-free cruises. With the right mindset and expectations, I think you will have a great time!