That is not correct. Water can be a poison just as fluorides can be a poison. Both are beneficial in some amounts and damaging, even deadly in other amounts.
“Water can be a poison just as fluorides can be a poison...”
Fluorides may be beneficial if you are running a concentration camp ala Nazi Germany and want to keep the victims docile as they wait their turn in the gas chamber.
I can live without fluorides. I cannot live without water. WATER IS NOT A POISON!