This is very helpful, thank you! I knew there had to be others who have traveled on Disney with car seats successfully. Everything about a cruise is foreign to me so thinking through the logistics of it all with kids is a lot.
Depending on the size of your 3 year old you could try a RideSafer harness. saferide4kids.com. They are expensive, but if you are having more kids and travelling a lot it might be worth it. They are even legal for use in the US in place of a car seat. Then you don't have to worry about car seats in port. My 3 yr old is tiny and it did not fit her properly, but I will try again because I hate travelling with car seats.