So, tooth enamel can be removed mechanically (overbrushing, injury...), but more typically it’s demineralized and breaks down. In the first case you’re shit out of luck, you’re unable to make new enamel. The good news is that deminerlized enamel can be restored if it’s not too far gone, which is what fluoride is trying to do in your toothpaste.

So no, there is a rebuilding process, but the kind of damage done by acidic substances (including the excretions of bacteria) can sometimes be reversed to some extent. The trend is usually erosion however, and moderate intake of something like seltzer or soda is at least an order of magnitude less than straight water. In addition, while water toxicity is a thing, you’d have to overcome a pretty strong urge to vomit. Short of that, water is doing you no harm, and will never contribute to the erosion of your dental enamel.

There's a new fluoride replacement called novamine that is apparently significantly better at re-mineralizing teeth.

Novamin is technically a desensitising agent by blocking tubules on exposed dentin (boney tissue below enamel). There is some evidence suggesting it may aid remineralisation in combination with fluoride, however it is far from conclusive.

A number of toothpaste brands in Japan claims to remineralise teeth with a hydroxyapatite based formula but the results are yet to be confirmed.

Dentists have been using zinc oxide gels and other material to seal exposed dentin but the results are mixed and eventually the filling will fail. Once cavities reach a certain size not much can be done about it.