My experience in Venice is that at times it is insanely crowded, and that you should accept the crowds as part of the Venice experience. While there may be half as many visitors in December as in July, that doesn't mean the lines at St Marks will be any smaller, nor the shoulder-to-shoulder walk along the waterfront any less jostly. Especially when a boat arrives.
In March 2011, I took this picture from the Bridge of Sighs:
Look at all those people! Right around St Marks it was always like that. The squares were full, and the lineups were long. (My guidebook recommended going to St Marks first thing, and we did, and when we came out the lineup had swelled to ridiculous lengths that we were very happy not to be part of.)
To experience uncrowded Venice, sure, go in the spring or winter (but watch out for aqua alta, the high water) but also plan your days, get up early, be prepared to wait in some lines, and keep an eye for the cruise ships. When they come through, that's your cue to go the places they don't, like the outer islands or just a few blocks inland from the Grand Canal.
Here we are just a tiny bit east of all that action (you can recognize the landmarks), headed for Castello and as you can see, we pretty much had the place to ourselves. That's the real trick.
Here's another picture taken very close to there. Not what most people think of when they think Venice, but undeniably uncrowded and lovely:
I had a wonderful (and mostly uncrowded) time thanks to the routes in a book called Strolling Through Venice, which I recommend.