Civitavecchia is indeed a long drive from Rome, so take that into account. But as said by others, these private companies can't afford not to drop you off on time - it will ruin their business.
As mentioned, request to get back to the ship early (like 2 hrs prior to the all aboard time) and stick to it. Chances are that if you are stuck in traffic - the cruise tour may be as well.
Take a picture of the port authorities info - in case you would be late - at least you can call someone
in the even that you would not make it, travelling to the next port is not the biggest issue, there are plenty of ways of transportation - but this will cost you money
If you can, stay in Rome/Italy a few days prior/after the cruise, so much to see!
Martine1969 got a reaction from MicDay in Getting off the ship at the Ports of Call