We had a lovely time in Venice, but wish to pass on some valuable lessons about transportation there.
1). Do not count on Alilaguna water buses to go to the airport. We bought tickets in advance, joined 10 other people at a stop on the morning of our last day and had not one, but two Alilaguna boats pass us by refusing to pick us up. This is the worst kind of delay to have when you have a flight to catch. One pulled in long enough to say he had to pick up a large group later and didn't have room for us. He promised another boat in 10 minutes. This was false. The next boat in 30 minutes zoom right on by - totally full - and did not even stop.
Our solution: take the next vaporetto to the Piazzale Roma and then catch an AirPort Express bus to the airport.
To Alilaguna's credit, they did give us a full refund to our credit card after I emailed a complaint and a photo of our tickets.
2). When flying out of Marco Polo Airport, allow AT LEAST 3 full hours (4 would be better) after arriving at the airport. We had no checked bags (saved us a line) and got our boarding passes from an electronic kiosk. But there was only one security line and it took more than 45 minutes to get through. That was only the first line. It turns out there is a second Passport Check line and it took more than a full hour! It snaked through the airport and moved at a snail's pace. Thank goodness we're early birds. By the time we got through passport check and found our gate they were already boarding.
We found the vaporettos very easy to use up and down the Grand Canal, and also for a day trip to Murano and Burano. The multi day pass was well worth getting.