Fluoride in the water
Whether or not fluoride is added to the water you drink, you will have dental decay if your diet is poor or you don't brush your teeth thoroughly and regularly enough.
And by poor diet I mean the diet of 95% of westerners - high in sugar and processed foods that stick to the teeth.
The only person I know with no fillings grew up in a family that avoided junk foods- a habit she has keep up as an adult (she's now 49 years old).
So I think the attention paid to putting fluoride in the water to improve the health of small children's teeth is almost certainly a red herring, as it focuses on an element that is insignificant compared to diet and tooth care.
As for the negative effects of adding fluoride to the water due to its neurotoxicity and other negative health effects, I think it wise to follow the lead of European countries and not take the risk of adding contaminated industrial waste (fluoride) into the water supply with the spurious hope dental carries will be avoided.