Before I stopped to think about it, the create water felt odd with not being potable, even in smaller amount.I'm not sure I see the relevance of the question, since purify food and drink needs to be cast on Supply and create or destroy water doesn't create Supply.
Thinking about it, it makes sense from the standpoint of supply being food AND water, that it wouldn't make supply.
And so, keeping with the simplified accounting of supply it doesn't really do anything to have it either potable or purifiable.
I'm imagining the case in the desert where someone sabotages the waterskins (or they decide to eat the camels) or at sea where the water barrels are bad but they can catch fish. Or where the way stop's well had bodies chucked in it and is fouled. (Or do any of the things that get you supply specify if it is only the food part, or only the water part? )
What changes as far as the supply rules if the created water is potable or purify food and water work in them separately (even if not "supply")? Does it make those story ideas doable?