A means of carrying water during your Tour du Mont Blanc is essential.
Despite the possibility of sometimes obtaining water en route we'd suggest that you always set off in the morning with at least 2ltrs of water each regardless of the weather.
During the hike we saw a number of water containers used. The one that looked the most unsatisfactory was a 2ltr lemonade bottle stuck in a rucksack side pocket. It unbalanced the rucksack, made a sloshing noise as the hiker walked, and being a transparent bottle, allowed the contents to warm up in the sun.
By far the best solution is a water pouch of the kind illustrated.
It will fit inside a rucksack, doesn't require removal when you want a drink, keeps your water out of the sun, and deflates as you drink which deters the water from sloshing around.
The only downside is the inevitable taste of plastic the first few times you use it.
So if you buy a new one, our advice is to fill it with water and leave it overnight a few times until the taste of the water is a little more pleasant.