Depending on the density of the weaving, the canvas may be waterproof without any treatment. My understanding is that the cotton fibers expand when wet, so the tiny pores in the weave close up. If the weave is dense enough to begin with, it'll resist water penetration

Some of my canvas saddlebags are naturally water resistant without a wax treatment. I have a Carradice bag on my bike and it will stay dry inside even after a day of steady rain.