There are several options.  you catch a land taxi for around €50 which takes 25 mins  you take the cruise lines own transfer which costs around £20pp  you catch the local ATVO bus to Piazzale Roma for €3pp and then you either walk or catch another bus to the port or  you take a water taxi which will be around €150.
There are taxis just at the exit of the airport building - usually plenty of them and from memory the journey time is about 20-30 minutes, depending on traffic. I can't remember how much it cost but it can't have been that bad or I would have remembered. Taxi drivers are very familiar with which ships depart from where and take you straight there without any hassle.
Do not take the taxis touting for business - only take those from the proper taxi ranks and agree the price for luggage beforehan.
I would check to see if transfers are included as I did this trip on the Marco Polo from Venice last year and coach transfers were included.