I always used plain old Glad freezer bags for backpacking. They're strong enough to serve as an emergency canteen or whatever. The generics are much less durable. For critical items, I would put one bag, sealed with a twist tie, into a second bag that was sealed the same way. Never had a problem.

For canoeing, I'd consider using my old Nalgene canteens in the same way. Sealed Glad bag inside, and close the lid. Plus, bonus points for extra flotation in the pack.