Travel from Sorrento to Rome

The bus from Sorrento to Rome is 3.5-4 hours, about the same as the train
The bus will not take you to the airport, it will take you to a bus terminal, and you will have to take a cab to the airport from the bus terminal Option would be to take a cab to the main train station, and the train to the airport, but this would take more time.

If you're not bent on pinching pennies, I'd propose that you (1) hire a car to take you from Sorrento to Garibaldi rail station in Naples; (2) buy a ticket on the NTV Italo train OR the Trenitalia Frecciarosa to Romeeither of which will take you a bit over an hour to get to Rome. (3) Take a taxi from the rail station (Tiburtina stazione for the NTV and Termini for the Trenitalia) to Fiumicino airport.

Personally, we would not do it. The timeline is too tight with too many opportunities for a delay unless, of course, you are willing to dire a driver from Sorrento to the airport. More important question is when does the ship sail?

Since you have not booked your Rome flight, you also have the option of taking the Curreri Viaggi bus from Sorrento to the Naples airport. Time 1H30m cost E10 Then fly directly to Venice on Easy jet. Time 1H15m starting around E 39.
Be aware of their strict baggage rules.

Traveling from Sorrento to Rome Fiumicino airport to catch a flight to Venice is a poor planning choice. The two most sensible options are: 1 - take bus or taxi from Sorrento to Naples airport and from there fly to Venice. 2 - take Circumvesuviana train or bus or taxi to Naples train station and from there take a Freccia high speed train directly to Venice. Only few trains require no change (many require a change in Bologna or Rome), but no matter which train you choose from Naples to Venice Santa Lucia (on the Gran Canal) is just over 5 hours. Either option will get you to Venice much faster than going to Fiumicino to catch a flight to Venice. That's at least 7 or 8 hours city center to city center if all stars and planets align perfectly.

First, you are looking at 3.5hrs-4.0hrs to get from Sorrento to Rome-FCO. Then, you have another 1.5hrs-2.0hrs (perhaps a bit shorter) for security check-in for your domestic flight to Venice. Then, you have another 45 minutes to get your luggage at Marco Polo and take the bus to the Piazzale Roma where you are close to your ship. That's about 7.0hrs worse case assuming your plane is on time. You can actually take the high-speed train from Napoli to Venice in the same amount of time as flying in from Rome-FCO. First, spend the 70 minutes to get from Sorrento to Napoli Centrale on the Circumvesuviana train. Then go upstairs and get onto the high-speed FS FRECCI trains from Napoli Centrale to Venezia Santa Lucia. Trenitalia shows one train change at Bologna. 5.5hrs to get to Venice. Also about 7.0hrs overall. From the Venezia Santa Lucia train station, you can walk about 3/8 mile to the Piazzale Roma where you can get onto the people mover out to the ship.

Larry, you forgot to add the flight time from Rome to Venice. Add another hour to your calculation. 8 hours is more realistic. There are also 2 daily Frecciargento trains from Napoli to Venezia SL. that require no change. One at 07:30 and one at 16:30. Both will take you to the Gran Canal in 5 hours and 5 min. In 5 hours from Naples you may not even have boarded yet at Fiumicino.

IMO, it's not a good idea to travel a long distance with several connections on the same day your ship will sail. Too many things could go wrong, leaving you high and dry and having the ship depart without you. Take a Circumvesuviana commuter train from Sorrento to Naples in time to catch the high-speed train that leaves Naples at 18:00 and arrives in Bologna at 21:37. Take the high-speed train that leaves Bologna at 22:10 and arrives at Venezia Mestre at 23:23 and at Venezia Santa Lucia at 23:35. Spend the night near Mestre on the mainland or spend it near Santa Lucia in the heart of Venice. Explore a bit of Venice in the morning and then board your cruise ship in the afternoon.

Totall agree with train versus air. May leave day before and relax for an evening and morning in Venice.

Thanks everyone for your help. I'm going to rethink our plans for travelling. Cruise starts on 27th August and they sail 28th August. I'm not sure what time you have to check in by. I'm trying to get in touch with the cruise line Norwegian spirit. Gai
Gold coast

Good idea to check with cruise line. I would never want to travel so far on the day of cruise departure, I would be too stressed. To me it is better to get there night before. Agree with others to use rail, not flights.

I vote for taking the high speed train from Naples to Venice. Since you know your travel date you can probably buy a cheap train ticket on the Trenitalia web site. My ticket was $32 for Rome to Venice.I got it a couple of months before my trip. It sounds like you board on the 27th and stay in port all night and do not leave port until probably the next afternoon. I had a cruise like this last year. I took the train from Rome to Venice and arrived around 2 pm. I went out the front of the Venice train station walked over the foot bridge to the people mover riding it and ending up at the cruise ship port stop. I walked to the ship check in where by this time (2:20) there was no line of people waiting to check in. I got checked in in 3 minutes then walked onto the ship with my bags. Very easy. So of course I think the train is a very easy way to go. I sat with and had a nice visit with some people from Brazil. Also it was great watching the Italian countryside and big name cities go by. And in the worst case of some sort of delay the ship doesn't leave until the next afternoon. So the worst that might happen is you miss a meal. If this is your scenario enjoy the ease and comfort of a great train ride and a no hassle check in.
The sail away from (or into) the port of Venice past St Mark's Square is one of the great cruise sail away experiences ever. Be on deck with your camera ready.

If your ship won't sail until the day after you board and you are given a range of hours to check in, then go ahead and travel from Sorrento to Venice on the same day that you board. Just be sure you get an early start. If one train were to break down, you could simply take the next one. Circumvesuviana commuter trains from Sorrento to Naples run at least twice/hour. High-speed trains from Naples to Venice run once/hour.