Undrinkable is a synonym of impotable.
Impotable is a synonym of undrinkable.
As adjectives the difference between impotable and undrinkable
is that impotable is (rare) not drinkable while undrinkable is not drinkable.
impotable
English
Adjective
(-)
(rare) Not drinkable.
Synonyms
* undrinkable
References
* Oxford English Dictionary , 2nd ed., 1989.