I recommend a great baby carrier to carry around your baby. They can easily fall asleep and take a nice nap as well!
Ergonomic Baby Carriers and Baby Wrap Carriers - Ergobaby
A hip carrier could also help carry around your baby or toddler around the cruise
Tushbaby: Best Hip Seat Baby Carrier For Babies, Kids ...
I know RC offers cribs and pack n plays, but if you’d like to bring your own it would be helpful!
Pack 'n Play Playards & Playpens | Graco Baby
Most ships only allow babies in diapers to play on splash pads and not in the pools, but good to have!
Swim Diapers - Target
Might be useful to bring your own. I would worry about glass plates and babies possibly knocking them over
Plates - Dishes: Baby Products
Don’t forget about snacks and containers to keep your baby happy and fed!
Snack Containers For Babies
Make sure you pack more than enough diapers! I know with RC, you can add a luggage tag to a box and get it taken to your room on the first day! Don’t forget those wipes either. Ships normally don’t have these items!
Pampers Diaper Box - Target
You may not want a huge diaper bag, so having a travel one would be helpful
The Getaway Bag by No Reception Club | Best Travel Diaper ...
Make sure your baby is drinking enough water throughout the day