We have a 7:20 AM flight out of Venice. Do we need to say on the mainland in order to get there on time? If not, what times do the boats/trains start?
With a 7:20 flight you probably want to be at the airport at 4:30 or 5:00 am or earlier. With that in mind I'd stay on the mainland. You don't want the hassle or the stress, even if trains do start that early (I'm not sure). When I had a 10am flight out of Fiumicino, I stayed in Fiumicino instead of Rome because I knew I needed to wake up around 6 anyways, and it wasn't worth it.
there is no need to stay near the aiport for a 07:20 flight although you can. Both the orange ACTV bus and the blue ATVO fly bus begin running to Marco Polo from Piazzale Roma at 05:00 and will get you to the airport before 05:30. You can walk or take the night vaporetto to P. le Roma. You can also take the Alilaguna shutttle boat from Venice to Marco Polo. Linea Blu leaves the SanMarco area at 03:40 and departs from the Fondamente Nove stop at 04:21. It arrives at the airport at 05:00 Current cost for Alilaguna is 13 euro per person. www.alilaguna.it Either works well and you can enjoy your last night actually in Venice. Frankly, there is not a lot going on at Marco Polo before 05:30, so either way you should be fine. If you still feel the need to be close, there are hotels very close to the airport.
If you want to stay close to the airport, Courtyard Marriott Venice Airport is not only close to the airport but also provides a complimentary shuttle service to/from the airport. We stayed there several years ago with our points. Looks like their prices can be as low as 120 euro/night. Have fun on your trip.
Ron explained all the options perfectly, getting to the airport on time for an early flight is perfectly doable. My vote is to stay in Venice. Spending your last night strolling through lovely Venice is a wonderful memory and a perfect end to an Italian adventure!
The tourist office has timesheets showing which buses you need to catch at Piazzale Roma for which morning flights.
Thanks to everyone for your responses. We have looked further into a high-speed boat and will use that so we can stay in Venice.