Fluoride in the water supply does absolutely nothing for tooth decay, if anything it actually damages the teeth. Fluoride is a mild sedative and reduces IQ (up to 15 points), it is actually a poisonous byproduct left over from a manufacturing process (don't remember which).

If you buy bottled water you should check the label to see if contains fluoride, if it does buy a different brand without it.

Sorry for the lack supporting links but I don't remember where my info came from, but it is all over the internet.