To what extent is fluoride absorbed through my skin when I take a hot shower?

Not to worry. "inhalation and skin absorption can be significant for some occupational exposure situations, but are not important exposure routes for Fluoride in drinking water." http://des.Nh.Gov/organization/commissioner/pip/factsheets/ard/documents/ard-ehp-14.Pdf.
Undetermined. Hydrofluoric acid is absorbed through the skin and once absorbed, this type of Fluoride is distributed throughout the body, with the highest concentration found in calcified. The extent of absorption needs to be studied further for each type of flouride that is put into the public water supply.