Like you we are flying in to Rome (4 days) prior to a cruise from Civitavecchia. We plan to take the train. We've taken many, many trains in Italy and have never had any problems.
Take a look at Italian Trains
. The little button for English is in the upper right corner. Try for a connection on the IC trains (not local) for a faster trip.
When are you cruising? We go June 12th.