I didn't know that plain bottled drinkingwater existed until I was abroad in my 40s.
When i was growing up, we never took water out with us unless going on a picnic..
A typical day's drinking for me through my primary school years would be a glass of milk with breakfast, a small bottle of milk at morning break, a glass of water at lunchtime, then tap water at home as and when I felt thirsty, which wasn't very often. Then, I think, milk at bedtime.
Once in secondary school I drank even less.
By that age, I think my parents assumed I was old enough to know when i needed to drink, and probably had no idea how little I actually drank - I don't think I had anything whatsoever to drink until lunchtime!
Basically, I only drank when i felt thirsty, which wasnt very often, and was almost always water (our local tapwater was good back then).