If you want to visit Rome from Civitavechia the ships excursions are both horribly expensive and tiresome due to the traffic
The best way in my opinion is to take the bus to the port gates(usually free)and walk straight ahead a few hundred yards to the station
You can buy returns to Rome which include the metro for around 10 euros
You must authorise the ticket before getting on to the train at the machine just as you exit to the platform-watch how others do it
The trains are frequent and quicker than road
I got details of the Italian rail system by googling-cannot remember the exact site but it took no finding
Enjoy the cruise-you will judging by the great itinery!