The best way to reach Rome is to use the train. Civitavecchia station is a shortish walk from the dock gates and the trains in Italy are good and cheap. Taxi's are very expensive and the buses inconsistent and slow. .... Nei.
The train fare to Rome last year was 9 euro return, which includes the use of the metro and buses in Rome. There is a station near to St Peters or you can get off at the Terminii. For train times see www.trenitalia.com. The station is about a 10-15min walk from the port shuttlebus drop off point in Civitavecchia, walking with the sea on your righ.
It is easy by train and I've given a detailed description of how to do it on earlier pages of this site a day or so ago if you want to look for it!!!!!
Thanks for your input and info very helpful,looking forward to doing the Rome excursion ourselves.