Give some thought to the canvas 'weight', or duck, which essentially is the thread size and indicative of overall canvas thickness. Typically the choices are lightweight #12, standard #10, and heavy #8. The weight of the canvas itself is negligible compared to the canoe's weight, but the thicker the canvas is the more filler, aka mud, it'll absorb, and mud is heavy. I believe most canoes 18' or shorter used to be supplied with #10 duck which I assume made the canvas more resistant to rips. I figure people these days are much more careful with their canoes, so I opt for lightweight #12 to make the canoe a little lighter.
You can treat the mud with a liquid algaecide - you'll find some at your local hardware store.