My husband and I recently took our 13 month old son on a Royal Caribbean cruise for 8 days. Although a little older than your daughter, I too was a little apprehensive prior to the trip especially since it was going to be just us three with no other help.
1. My son was eating table food at that point, but I will say that they should be able to steam vegetables for you since most of the formal dining meals came with some sort of vegetable. Also for us, they seemed eager to help make our son comfortable. Otherwise on Royal Caribbean you had the option of eating buffet style in the non-formal area and that way you can kind of pick and choose some foods for her that you can mash up yourself. I do not know if you are flying to port or you live near the port, but if flying depending on how long a flight, you might just want to bring some jarred baby food (even though I know it's not great) because it is difficult to find anything for them to eat until you actually get aboard the ship. Once aboard, I also made sure during breakfast hour to take some extra boxes of cheerios, fruit (fresh banana, etc) and yogurt. I stored stuff in the mini-fridge in the room so that I always had stuff available.
-->We had plenty of high chairs on the ship and it was not a problem for us, but during breakfast and lunch we still brought our stroller so that we could both go through the buffet line together and not have to take turns while someone held/sat with the baby.
2. Make sure to confirm with the cruise line that you need a pack and play. I ordered it initially when we booked the cruise and then called back a few days prior to the sail date to reconfirm it only to find out the order had never been placed. Then once we got aboard the ship and went to our room I noticed it wasn't there. I immediately called our stateroom attendant and notified them and by the time we actually set sail, the pack and play was in our room. You just have to really stay on top of them with it.
3. Other things I found helpful:
---> Inflatable infant swimming pool. I put this in the bottom of the shower in the room (Royal Caribbean did not have any bath tubs) and used this to bathe my son. It worked great! It also did double duty on the pool deck as his own private swimming pool since he was not toilet trained to be able to use the cruise pools. He absolutely loved it!
-->Cheap stroller. Do not bring your best one with you as it most likely was expensive and also is pretty bulky. We bought a stroller specific for the trip itself. We ended up getting the Especially for Kids Trend sport stroller that was $49 because it was able to recline so that he could take a nap (came in handy for us to still be out at bingo or on the ship during nap time) and also had cup holders, sun canopy and good size basket underneath for things. It also folded up so well we could store it in the closet in the room easily out of the way.
-->Disposable bibs: just easier than having to wash bibs out all the time
-->Baby spoons: the ship spoons were too big for his little mouth.
-->Pack plenty of wipes/diapers (more than you think you'll need). Not sure if the ship has any on board, but even if they do I am sure they will be expensive and no guarantee they will have your right size. I pretty much spread out the diapers and wipes when packing into different bags.
-->Although we did not need it our pediatrician recommended to bring Infant Tylenol, Motrin and Children's Benadryl just in case something happens while on the trip. You might check with your own pediatrician for any recommendations.
All in all, everyone was very helpful and nice to us, even passengers. I cannot tell you how many people came up to us and said that they missed their son/daughter who they left at home and wished that they had brought them after seeing us on board. My husband and I had a great time and our son had a lot of fun as well. We would definitely do it again and I think the key is to be as prepared as you can be and be flexible.
Hope this helps. Have a great time!