At what point does it become dangerous to continue to drink water ?
According to this page [0], the LD50 of water is 6 liters, so somewhere before that.
[0] https://www.compoundchem.com/2014/07/27/lethaldoses/
Well, that's something I never heard about, and contradicts even basic high-school chemistry.
Where did you read it?
https://www.atsdr.cdc.gov/phs/phs.asp?id=210&tid=38
Fluoride is one of those ions that diffuse thought and out of our bodies almost like there is no barrier anywhere.
There is also water intoxication, which can be deadly.
According to this page [0], the LD50 of water is 6 liters, so somewhere before that.
[0] https://www.compoundchem.com/2014/07/27/lethaldoses/