We did this two week back to back cruise from Venice, departing on the day our flight from the US arrived in Venice. Our flight was delayed in landing by 45 minutes due to fog. We later learned that we were lucky-some flights were diverted to other airports and the delays were several hours long. If you are flying to Venice this time of year, apparently fog can happen due to the cooler air and the warmer ocean temperatures. In this case it did not cause us any problems.
At first glance the options for airport transfer to the port looked a bit difficult, but in the end it went smoothly and inexpensively using the bus. They run frequently both to and from the port, and the buses going back to the airport at the end of the cruise start as early as about 4:30 am. It takes about 30-40 minutes. When arriving at the port, the People Mover is right there and it takes you to the port itself. It is a small automated train and costs 1.5 Euros each way, The bus was 15 Euros round trip. Taxis and water taxis are considerably more expensive.
Check in, embarkation and debarkation were extremely quick and easy. In fact from the time we disembarked the ship and exited the terminal was only five minutes. There was no passport check and we walked right out the door and to the People Mover.