Sammee- You have not received a response as of yet because you probably should have posted your question directly in the European cruise section. but let me try to help- You certainly could hotel in Civitavecchia and do what you are proposing. However, I'm not sure it buys you much. From FCO you still have to take a train to Rome Termini to get to the train that goes out to Civitavecchia. There is no direct train. So you add a cost of a round trip ticket to Rome for each time you would want to go into Rome (assuming that the extra ticket to Civitiavecchia would have to have been purchased anyway)
Getting to FCO for your return flight requires you to travel into Rome/change trains and then go on to FCO. So I think in the long run, just staying in Rome is the best option as there really is not much to see out in Civitavecchia anyway.
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