This graph lines up the individual states in the United States beginning with states that receive the least fluoridated water to states that receive the most fluoridated water.
You can see that states with more fluoride show no difference in tooth decay from states with almost 100% fluoride.
While there is some low quality evidence which indicates that fluoride in water may slightly reduce tooth decay, no medium or high quality research has shown that fluoridated water is safe or effective.
Note: The actual chemical used, Hydrofluorosilicic acid and Hexafluorosilicic acid, have not been tested for safety or efficacy at all. They are believed to break down into the pure Fluorine ion when introduced to the public drinking water.