Duluth canvas packs are of course not waterproof. Nor is a hiking backpack. You must use a waterproof interior liner, which will also provide air pocket flotation for the pack. Most people use a tough, clear plastic liner, such as the ones
Duluth itself sells, or a more expensive breathable liner such as the Granite Gear eVent liners I use.
Here is a video of how to pack a Duluth and, at the end, how close the top of a poly liner. You can also twist the top of the liner together (instead of rolling it), fold the twisted part over, and cinch the fold with very strong rubber bands or bicycle tube.