Train to Venice from Rome cruise port of Civitavecchia.

It depends on how much of an independent traveler you are. It also depends on how much luggage you are carrying.

I've landed at Civitavecchia and taken the train to Rome Termini. The cruise port is several miles from the train station. The easiest way would be to hire a taxi from the port to take you to the train station. (There's a cheaper bus shuttle to the station, but if you have a lot of luggage it would be inconvenient). Then take the train to Rome Termini, where there is a LONG, LONG walk from the platform to the main part of the station. From there you would get a high-speed train to Venice, either Trenitalia or Italo.

There used to be a private bus that would take you directly from the cruise port to the Rome Fiumicino airport, but I don't think it is running anymore thanks to COVID. A cab to the airport will be quiet expensive, but it's fast and convenient. They you could fly to Venice.

When you come into Civitavecchia, you'll have to catch a bus up to the front of the cruise port. From there, you catch another bus about 2 miles to the train station. Warning: you'll often find the lines long to get on the first bus so move as fast to get off the ship as possible.

During the week, trains going into Rome are about every hour. If you see a train, move quick to get on it. It's maybe a 40 to 60 minute ride into Rome, and the main Termini where you change to a different train to Venice. You can buy one ticket there that takes you all the way to Venice.

There is a train going north out of Civitavecchia, however it goes to Pisa and there are 4 connections/stops to be made. It's just easier going into Rome Termini.