I don't think there are many cruises taking this route, and it would probably cost more than hotels and trains. Our Sunday newspaper travel supplements are full of ads for cruises, and I can recall seeing this journey only as part of a longer cruise around the Mediterranean.
You probably need to talk with a real bricks-and-mortar travel agent.Edited: 11 May 2018, 17:46
The route proposed here does not make sense geographically. To get from Rome to Venice by sea (or vice versa), you must go round the south of Italy. Quite a lot of the trip would be along Italy's east coast. Florence and Cinque Terre are in the wrong direction, along or near the west coast. Italy's geography actually limits your cruise options.
Thanks Sushi, do you remember which cruise?
You can also search here: cruisett.com/content.php…
That being said I would not recommend such a cruise. So much is inland in Italy. And such a cruise would not get you to Florence or the CT.
Maybe Costa, MSC, and others. I look them up on Vacations to Go. But geographically, they usually don't go in the order of ports that you listed.Edited: 12 May 2018, 02:46
We're have a cruise booked that starts in Rome and goes Southbound towards Naples and Sicily, and around to the Adriatic, stopping in ports at Montenegro, Croatia and Slovenia, and then ends in Venice. It's through Windstar... A much smaller ship, but it allows us to see ports that many of the bigger ships can't access.