The train is the fastest and most convenient way for most cruisers. If leaving from Termini station, be aware that it is a long 15-minute walk from the front of the station to the platforms for the train to Civitavecchia. When you arrive in Civitavecchia, take the city shuttle direct to the cruise port transit center--it's cheap and runs every 10 minutes. However, you can't buy your shuttle tickets from the driver--buy them at the train station cafe or the station travel agency desk (which calls itself "Tourist Information" even though it is really a travel agency). Bon voyage.