Venice has a very limited off-season, which includes now, the lull before Christmas. It's really only ever more quiet in early-to-mid January. Even now, though, the crowds flock to visit the heart of Venice, the Piazza S. Marco, with the basilica, the doge's palace, the Museo Correr, and the classic view out over the bacino.
The piazza is normally thronged year-round and day-long with visitors and pigeons. Thus acqua alta creates a rare opportunity, especially if one has come in knee-high wellies: to experience the piazza without either tourists or pigeons. Standing ankle-deep in swaying water, the vast space around me in highly-coordinated hues of white, pink, and maroon stone, the tourists marginalized to the passarelle, except for the few who came more prepared, the piazza was, for once, a very tranquil experience. (The photo above is a better photo of people on passarelle in front of the basilica, but wasn't taken from the most peaceful part of the piazza.)