Try this site which has a journey planner for the vaporetti: http://www.actv.it/english/home.php

When in Venice last year I stayed near St Mark's Square. From the airport I took a private water taxi because I'd just missed the public vaporetto and there wasn't another for an hour.

It was good fun whizzing along the canal with wind in hair feeling a bit like a movie star, but it was 90 Euros, so definitely not cheap. I took the public vaporetto back to the airport which was much cheaper but I think it took an hour or more. It was mid afternoon so can't help you re early morning service.