As others have noted... external packs can work fine on trails. They aren't as good cross country when you want something with better balance and that is less likely to snag on things.

The other think to keep in mind is weight. The typically external frame pack is 5lbs. While there are internal frame packs that weight this much or more, it's easy to find numerous good internal frame packs that are significantly lighter. Saving a few pounds might not seem like a lot when canoeing with short portages, but when you are doing 10-20 miles... and makes a real differences. Depending on your style of canoeing you might be looking at more than a new backpack because you might find your canoeing gear heavier than you will want to carry long distances.

--Mark