Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
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!