The water in this area has fluoride naturally, the way lower levels of tooth decay are evident. I've only been to a dentist 3 times, one of those when I had chipped a tooth slipping on an icy patch, another when I broke a tooth on something hard. I've never had a filling. I've noticed in recent years how frequently people need fillings renewed, suggests to me that a lot of dentists do substandard work so they can charge for doing the same job many times; much like surgeons that do things like "partial hysterectomies" then 2 or 3 years later have to remove the rest, and charge the same both times as they would if taking it all in one operation (doesn't usually happen in NHS hospitals)
I'd be a lot more worried about the other things that get added to water; when you live near the mouth of a river how many bodies has water been through before it gets to your tap?
As for buying bottled water a lot of that is a con, much of it comes out of an ordinary pipe and gets treated (maybe just filtered or very mildly flavoured, not water direct from a spring. A few years back one of the big name soft drinks companies started selling bottled water - it came from a pipe on the normal domestic supply in a part of London, one of the workers blew the whistle.