According to Wikipedia, the Grand Canal in Venice is 3,800 m long, 30-90 m wide, with an average depth of five meters (16.4 feet). The depth of the water in the canals will vary from canal to canal but is usually around 10 ft deep.

http://en.wikipedia.org/wiki/Grand_Canal_(Venice)

Exactly! I was about to say there are variations in the water depth depending on where you are; even the same canal can have dips and shallow areas and the locals get to know these and navigate accordingly.

The water is VERY dirty, it smells bad, raw sewage goes into it :( if you fell in you’d probably want to die so no need for a life jacket! :)