Many walkers, long stages, thin services between towns, and fountains whose reliability travels by word of mouth and out-of-date route notes: pilgrim routes concentrate exactly the water problem every long walk runs into.
Fontavio maps drinking fountains, toilets, showers and laundry across Europe, US and Canada, including the countries the major routes cross. It is a general map rather than a route guide, which means it does not know your stage plan, but it does know what is in the villages along it.
Offline matters here more than almost anywhere. Download the country before you start and the map keeps working through the stretches where there is no coverage and no reason to expect any.
It is also the sort of map that improves fastest when walkers use it. A fountain confirmed as running in July is worth a great deal to whoever walks past it in August, and confirming one takes a single tap.