Flouride should not get added to water in the hopes of getting dental benefits. The compositions of fluoride is not sufficient to have any kind of positive effect in regards to improving dental health or dental hygiene. Chlorine that is already added to the water is the only chemical benefit.
Current science determines that fluoride mainly works on the surface of teeth not from swallowing it. There are also negative bodily effects with unconrolled doses of fluoride to at-risk groups like renal patients, infants and those with thyroid problems. There is no known gold standard research proof of absolute safety.
See Connett et. Al 'The Case Against Fluoride'.