I don't know what is used on Ms B's rucksack, but I must comment that my waterproof (allegedly) camera case has a waterproof (allegedly) zip around it, and this is much stiffer to do up or undo than a conventional zip.
I use the word allegedly because I still don't know how waterproof it is, or isn't. In the 18 months I've had it, I have miraculously avoided being out in a real soaker, and when that time does come I have a plastic bag folded in the case ready to wrap the camera in, because I'm not taking any chances.