Water is widely available either from drinking fountains or from bars/restaurants in the villages. Unsafe water sources are marked as "non potabile." Except on a few stretches it is not necessary to carry at most 1L of water, as long as you refill the container when you have the opportunity.
Another reason for having a guidebook is being able to know the places where you need to carry more water.
Well worth knowing - ALL cemeteries in France have drinking water and you will walk past a lot. I was a bit wary at first of drinking cemetery water but I saw a lot of french walkers doing it, was directed to cemeteries by locals when enquiring about water and I must admit the knowledge came in very handy on the warmer days!