I am once again saying: Why not hydroxyapitite? A form of calcium, same as our teeth/bones. It even has water management uses because it adsorbs other stuff like fluoride and lead, which actually makes finding info about its addition to water for the purpose of teeth health difficult.
Though it seems like hydroxyapatite water would also make fluoride toothpaste even more effective.
The mineral ions introduced during remineralisation restore the structure of the hydroxyapatite crystals. If fluoride ions are present during the remineralisation, through water fluoridation or the use of fluoride-containing toothpaste, the stronger and more acid-resistant fluorapatite crystals are formed instead of the hydroxyapatite crystals.