The 24-, 48-, 72- and 168-hour passes are valid for periods of the specified number of hours (not calendar days), so calculate the hours carefully to avoid paying too much. For example, if you arrive at 10 AM on a Tuesday, a 72-hour pass will take you all the way to 10 AM on Friday.
No matter what form of payment you are using, you must tap in before going out on the floating dock, including for each vaporetto on a trip requiring one or more transfers. The validator will display a green check mark (currently) if the tap has registered. If you fail to tap in,el you are likely to be fined if an inspector comes along.
Be aware that vaporetti can get so crowded that not everyone waiting at a dock is always able to board the next vaporetto. If you're close to running out a timed pass on your last day, don't cut it too close.