I read part of this British YA novel in the '70s or the '80s. I am almost positive it was called 'A Drop of Fresh Water' or 'A Drop of Pure Water', although I have searched the internet for both titles and come up with nothing. The story: Society has broken down and the people of Britain are living a primitive existence. The two main characters - a boy and a girl - work in a field raising food of some kind. The water provided them contains a drug, but their minds are too clouded to know it. One day they find a stream of fresh water and they begin to drink from it. The drug eventually wears off, they realize what is happening to them and they take off on their own. I don't know what happens next because I didn't finish the book! How it ends has been bugging me for years!
The Awakening Water by G. R. Kesteven.
I'm sure I have a copy but I cannot find it so I can't provide quotes. However the summaries I find by Googling confirm it's the book I remember e.g. Google Books says:
"She held out the bottle to him. 'It's only water, and it's not doped.' John recalled the phrase and, as he tasted the cool, earthy water, he realised that not for three days had he drunk from his meal-time beaker, and during those three days a bewildering change had come over him. It was as if he had woken from sleep not to the usual plodding work, but to a multitude of new experiences and - strangest of all - to an awareness of himself.
As do the reviews on Goodreads. It was published in 1977 so it neatly fits the time you remember reading it.