Local is better than State. State is better than Federal. If this was going from the federal level to the state, I would agree that it's a win for personal liberty. Unfortunately it's local going to the state level. If a water district of 100,000 people all want this, they simply can't do it. It doesn't seem fair that people who won't be impacted get to decide.
I agree. When issues require collective action, it should be decided at the smallest capable level.
In this case, the smallest capable level is the individual. To extend your comparison: individual is better than local.
This _isn't_ an issue that requires collective action; people can treat their own teeth with fluoride or take fluoride supplements.
Personally I see states as an artifact of the 20th century. Why can't municipalities govern themselves? Why shouldn't my city be able to leave my state and join another. States made sense before the internet and communication was difficult. Now they're just a middle man without a lot of value.
If eastern towns of California don't want to be grouped together with San Francisco or Los Angeles: why should they be forced to be? Because someone in the 18th or 19th century determined it?
That 70% can use floride toothpaste or mouthwash. What happens to the 30% who don't want it in your scenario?