From my experience now living part-time in a small Portuguese town, and travelling around a fair bit--
Most towns have several permanent "fontes" in obvious locations in the towns--in squares, etc. I think it's safe to say they pride themselves on each of these being part of the town's drinking-water infrastructure, the same as the water lines into people's houses. Remember that indoor plumbing is often just 30 to 40 years old in many of these towns--pre the 1974 revolution things were pretty primitive outside the major cities. And so everyone used these public faucets. (In the VERY smallest towns you may still find public laundry sinks near the faucets where everyone did their laundry in earlier times. These seem to have been preserved as part of the public heritage. They sometimes have been repainted quite decoratively.)
Back to the drinking water: A specific example in our town, on the route through Vila Nova da Barquinha between Golego and Tomar--(a particularly hot, dry area--going up to 25 C / ~80 F this March weekend!) This is on the Portuguese interior route, about 4-5 days north of Lisbon.
I specifically asked an employee of our Junta Freguesia (town hall) --which is right beside the Caminho route that we see from our back window--and he assured me this water was potable...
As you walk north through VNd Barquinha from Golega, you cross the railway tracks following the Rua da Cardiga, then walk northwest, following the yellow arrows on the Rua do Rebeiro de Maia which turns into the Rua da Nova Escolha (street of the new school), for a block, past the small Fatima shrine on your right, to the Rotunda (roundabout), and then head up the hill on Rua dom Afonso Henriques towards Atalaia.
As you go through the roundabout, you'll see a notice board, one of the ubiquitous garbage bins, and a faded aqua wall to the right of your route. To the right of the wall (looking uphill, you'll see our lovely chapel), is a longish concrete bench with wall behind, with a faucet in the middle of the wall. This is good drinking water, I am assured by someone who should know.
(Also, if you need to pick up some cash, the Junta Freguesia (town hall) building to your left as you enter the roundabout has a Multibanco ATM that takes North American debit cards.)
And--if you want a quick break with a cup of coffee, or excellent baking, or soup, or a beer or wine...
If you walk up the very short hill (block and a half) towards the chapel (on your right/east) to the cross-street just in front of the chapel, and turn left, and go ~6 doors up, there is a very nice cafe with proprietors who speak some English. Highly recommended! A sheltered outside space as well as inside the cafe. Quite newly built in an old building.
A good place to stop about halfway between Golega and Tomar.