Venice can get very crowded, and visiting during the offseason or shoulder seasons can give you a better chance of exploring Venice without being a part of pedestrian jams in the small streets.   The city gets crowded not just with visitors staying in hotels but with day trippers from the cruise ships.  As the ships empty, the streets fill up fast.  One tip is to check how many cruise ships will be in the Venice harbor.  Cruisemapper is a website that can help  you plan when to arrive.

http://www.cruisemapper.com/cruises-from-Venice-4105/calendar

Good luck and happy travels.