From the port of Civitavecchia (west of Rome) you can take a local train ride for less than $20 return. Each way will take you about 80 minutes...enjoy.
I did this from Civitevicia last November. The quickest, cheapest way, is to go to the local train station (I cruised with Princess and they laid on a bus service to the edge of the port). From the port it's about 10 minutes walk to the train station. It's a fair treck to the centre of Rome, but some train services go direct - and it was only about 10 euros return. When you get to the central train station you'll be about 10 minutes walk from the collusium and there are lots of taxis and local/tour buses at the terminal too. I wouldnt recommend taking a taxi all the way as it's a fair way to go.
Also the ships generally lay on a coach service to Rome, but this is quite expensive in comparison to using the local transport syste.
Take the train. 10 euros return. Trenitalia web site will give you the times. Get off at St. Peters station for St. Peters and the Vatican. When you have finished take the bus (No 78 I think) which will take you to the Colliseum ( 1 euro from the shop by the bus stop). You can then walk to the Roman Forum, Trevi Fountian and the Spanish Steps.From there it is at least a 20 minute walk to the main train station and allow at least 15mins to get to the platform for Civtevicia.Take it easy there are plenty of stops for refreshment. Take a good map!
Have a good trip
Chris & Val Ashwort.
The port (Civitavecchia (or similar spelling!)) is quite some distance from Rome. A bit like the distance Southampton is from London. We used the train. It's a 15-20 minute walk from the port gates to the station (forget about taxis - they just want to take you to Rome, for a fee) The trains are clean, fast and reliable (An achievement thanks to Mussolini!)
Take a look at the Trenitalia website for timings, cost etc.
Do remember to validate your ticket by punching it on one of the machines located discreetly on the platform prior to boarding or face a stiff fine, even if you just bought the ticket 5 minutes ago from the ticket office!
Enjoy Rome or the itsy bitsy bit of it you will see in a day. Hopefully, you will enjoy it so much you will revisit again and again. Ryanair are doing £20 returns at the moment!
Like all the rest train is the best , when you get to rome use the buses to get around , take great care of bags and valubles pick pockets all over
even the locals will warn you when they are about , one day is not enough to see rome but its enough to make you want to go back, enjoy it .