6 km sounds like an awfully long portage and you will not want to be wearing soaking wet boots for that. And where did you find such a heavy canoe? My guess is that will be a two-trip portage because a lighter kevlar boat and a portage pack are bad enough to carry at the same time. If you need the stability of boots I would drag everything out of the water and put them on after that. Or else stick with trail runners or light hikers with no membrane. Leather does take a long time to dry but some of the synthetics out there can really take a beating without showing too much wear and tear. Depending on where you canoe the terrain can go from a walk in the park to boulder-hopping, which would require some extra support and stability. Last time I went to Algonquin I ended up leaving my boots in the car and either wore my trail runners or my Chaco's the entire time. If you are able to I would really suggest renting a kevlar boat from Algonquin Outfitters. I spent a week with a kevlar Swift Kipawa and it was a great boat.

If you plan on stepping in knee deep water, then don't get any GTX boots unless they are knee high. heh. Once GTX boots get wet, they take a long time to dry.