Hi Renata, I never bought bottled water on the Camino. Just filled up my water bottle in refugios and at fountains. All but a few fountains have drinkable water, and those that don't are clearly marked. There is no need to by bottled water, unless that is your preference.
Bottled water is easy to find along the way. I bought some quite often. Supporting the local economy, and being generally sceptic about tapped water. Besides, you you use the same bottle for too long, it becomes quite dirty. But the times I used water from taps along the way, wheter it was in the refugios or the taps for pilgrims along the way, I had no problems, even if my stomach easily gets upset when I am being abroad.
I kept a few Imodiums just to keep certain, but I never used them.
We walked the camino in October and met a French lady who told us that she got infected after drinking the water along the camino and advised us against drinking non-potable water. So we took our advice and bought bottled water along the way