Vaporetto validation

You have to tap it on the pad every single time you board or face a whopping fine. (It beeps and flashes green, if memory serves, but it doesn't actually stamp anything.) I think it's so they can more easily detect people who are getting on board without a valid pass. The situation is completely different on other mass transit systems in Italy. For example, once you buy and validate a transit pass in Rome, you don't have to keep validating it.

Michael's got it. It's akin to a bar-code at a self service check out at the grocery store. You just scan, it beeps, and you carry on. I used my pass about 20 times over 3 days, and was only asked about it once. That being said, I WOULDN'T risk not swiping it, you don't want the hassle of the consequential fines.
Do the first run vaporetto from the train station all the way to saint marks square early in the morning, and bring along the downloaded Rick Steve's audio tour. It's so nice to be on the canal before its crowded, and to hear the magical history of all the crumbling and restored palaces you go past. Stop at Rialto Markets while they're still setting up, and be at San Marco before 9 when the Campanile bell rings, and they first start letting people in. It's a different world before the crowds. Enjoy!