5 km outside of Muxia where you come down the forest path and first see a beautiful beach and the sea on the left the path has a rock wall with a water pipe pouring clean water. I drank from itband survived to do another full Camino.
there are two fountains. one just before morquintián (with a lovely stone cross nearby) and one after xurarantes (on the descent to the main road which you then follow into muxía, as @Colette Zaharie mentiones).
there were some drink machines in the first village after lires, frixe, which should also have a cafe and a small shop on the main road about 600m off the camino.
You can eat something in the cafe in Lires if you're hungry, no need to carry food.... You can also replenish your water there, of course. Oh, and a stamp for your credential if you want the Muxiana certificate