Hi! I am completely with you in regards to car safety. Our little ones are now 4 and 7, and we have traveled with them extensive both internationally and here in the states... and always with their carseats. We never check our carseats. We use Britax bags with wheels to transport the seats when we travel. Usually it is my dear husband lugging them through the airports but now my oldest will drag her safety seat

. On planes we will gate check the carseat/now booster for my older one but we have always taken my younger ones car seat on the plane as she does so much better and will often nap in it. I have used CARES in the past and it is not conducive to sleep like the car seat is. We also will not rent car seats. When we sailed out of PC, we have tried hiring Mears/limo/car service and we put our own car seats. Most recently we tried the Disney transportation, but we still had our car seats with our luggage for when we arrived in England (it was a transatlantic). I have stored the carseats under the bed (that was actually a high back booster), the shower, and on the transatlantic we were able to store them in one of the closets as the stateroom was an accessible one and the closets were quite spacious. When we used private transportation to the cruise, we arrived at 12:30 which was a really sweet time to arrive- not busy at all- and just dragged those britax bags on ourselves, but our room was ready so we put them in. Another time, we did just lug them around until our room was ready. We just sat and ate, but at least I was assured they were not in the pileup posted by Princess Schmoo. For the same reason, we do not leave our car seats out with the luggage the night before disembarking. We carry them off ourselves the next morning.
Enjoy your cruise!!! You will have a blast!