It's nearly always possible, if you choose, to forget that Venice is a real city with real people living in it. On the route back to the hotel of an evening we had to pass through perhaps the only Campo in Venice where loud music was being played and young Venetians were drinking and socialising very loudly: it was a nice reminder. There's a certain undercurrent in Venice, a kind of dull ache of political unrest, or at least reluctance to be governed, and one of the ways it shows itself is in the dirt, litter and graffiti which cover the city. As a tourist you have an option to allow your eye to float over it and most people, I'm sure, can do so. It's easy when dazzled by the shocking beauty of the Grand Canal to ignore the fact that the Rialto Bridge which you are standing on to sightsee is actually pretty shitty.
I was fascinated by the graffiti on the walls of Venice and the above is a small sample. The rest of my photos of graffiti, fly posting and so on can be found in this Flickr set.