As this word is neither listed in the online Merriam-Webster dictionary nor the Oxford compact dictionary (I don't have an unabridged copy on hand), I am somewhat suspicious that it isn't actually part of those "official" works. But usage determines the word in English, like all "natural" languages. Unpotable is simply the opposite of potable, i.e., water which is not fit for drinking. No doubt with the increased usage of the word, we shall soon see it grace the pages of college dictionaries everywhere.
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