@Teresa I agree, it's a modern system that makes things easier for visitors. Some people are still reluctant to use it, saying they don't trust the system to calculate the fare correctly.
@KRS From my experience, I'd say it's pretty simple to use. It's not even a new concept, many cities already have something similar.
@nancys8 The vaporetto validators are always on land. By the way, you don't need to tap out when using the vaporetto.
One corner case with tap&pay - at least to my mind - is what happens if a vaporetto stop doesn't have a validator. I remember with paper tickets, you had to manually write the date and time of boarding on the ticket in such cases.