My question is- Where are you going to find a taxi that can take 2 adults and still have space for three car seats? If you are going on a bus, I don't think the car seat law applies. Not sure of a taxi. But even if you brought them, I don't know how you would fit them into a taxi. With both of you and your luggage.
I would not worry about storing them once you get to the ship, but the hassle in bringing them is a bit much.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007