you just take the bus to Verona Station.
From there the train.
Check train schedule with FS Italian Railways.
The central station in Verona calls
VERONA PORTA NUOVA
For Venice you must use the Italian name
From one of the Stations: Mestre or Santa Lucia the way to the cruise terminal is well described on the Costa Website.
For cruise guests arriving in VENICE by train:
We recommend that you get off the train at the Venezia-Mestre station. From here you can take a taxi, by land, reaching the port in about 15 minutes at a cost of approx. Euro 25.
To reach the port from the S. Lucia train station, you can either take a taxi-boat (cost approx. Euro 60) or a free shuttle bus organised by Costa, which is available for the entire duration of the call at the port, and leaves Piazzale Roma every 30 minutes. Piazzale Roma can be reached from the S. Lucia train station on foot or with one stop of the waterbus. The Cooperativa Portabagagli di Venezia offers a luggage transfer service from the S. Lucia station to the Ferry Terminal, at the cost of Euro 5.00 per luggage item. If you wish to use this service, contact them directly inside the train station. In order to ensure that your luggage is correctly delivered to your cabin, please make sure that all luggage items are correctly labelled with the Costa labels that are included with your cruise ticket. If you do not have these labels with you, we strongly recommend that you do not use this service.
More convenient: rent a one-way-car from the airport and return in Venice.
Have a nice trip.