It is now widely accepted that the swallowing fluoride during tooth development, while it may make tooth enamel slightly harder (and consequently more brittle), does not make it more resistant to plaque acids. Any caries reducing effect of fluoride is topical (direct contact) and not systemic (swallowing it).
Fluoride ions present in saliva can slow down the loss of calcium and phosphates from the white spot lesion, the first stage of tooth decay.
The levels of salivary fluoride required for this to happen occur after using high concentration fluoride products such as toothpaste (usually 1,000 ppm), and revert to baseline levels after a few hours.
water fluoridation (at 0.7-1 ppm) raises the baseline pooled saliva concentration by only a minimal amount, inadequate to have any effect on the caries balance. Therefore there is generally no benefit from ingestion of fluoridated water
Furthermore, for salivary fluoride to have any effect, plaque must be present on the enamel surface in the area of the early cavity to retain the fluoride ions from the saliva, otherwise they cannot interact with the white spot lesion.
the fluoride ions interact with the outer layer of the plaque first, and penetration to the enamel surface is severely restricted when the plaque is thicker than around 0.5mm.
fluoride in concentrations above 2,000 ppm can damage the acid- forming bacteria in plaque (streptococcus mutans), and these concentrations are used in fluoride varnish and high strength toothpastes used in caries prone patients.
references: Lynch, RJ.M, Navada R, Walia R: Low-levels of fluoride in plaque and saliva and their effects on the demineralisation and remineralisation of enamel; role of fluoride toothpastes. International Dental Journal, 54-5 Suppl 1, 2004.
Zero DT, Raubertas J, Pedersen AL, Featherstone JDB: Fluoride Concentrations in Plaque, Whole Saliva and Ductal Saliva After Application of Home-use Topical Fluorides. J Dent Res 71 (11): 1768-1775, November 1992
Featherstone JDB: Prevention and Reversal of Dental Caries: Role of Low Level Fluoride. Community Dent. Oral Epidemiology, 27; 31-40, 1999
Robinson C: Fluoride and the caries lesion: interactions and mechanism of action. European Archives of Pediatric Dentistry. 10 (3)136-140, 2009