600 Denier are about the best for canvas. I had my glass boat covered with one for six years before last winters snow tore through it. I also had one of the poles with four straps to keep the water flowing off of the boat. Worked out very well.
I had a blue tarp on my tin last winter, not a good idea because I ended up with blue plastic dust everywhere and the tarp tore after a few months.
I cleaned out my lower shed (18x36) a few weeks ago and the boats snug in there now. I had thought about getting a tin carport to store it under as well.
I just replaced my boat cover for my v-hull. First I tried a cabelas cover, but it didn't fit right so then I found this place that had the same cover I originally had. I'm happy with it,