Al Timon

Easy drinking by a cool canal
Tipple of choice: The grappa is often free if you eat here as well.

Although primarily a restaurant, Al Timon is also a great place to drink by the canal. There's a fantastic outdoor seating area, purpose-built, and away from passing tourists; the nearest Venice gets to a beer garden.
Plenty of locals pop in after work to decimate the early evening "cicchetti bar", which is laden with all kinds of goodies. The staff like to chat to customers, and the drinks are relatively cheap, with a particularly good selection of spirits. They'll also stay open for as long as there's someone drinking.