"Coffee is not water."
Translation:Koffie is geen water.
Dat is inderdaad fout.
"geen" is used to negate a noun that, if not negated, would be preceded by "een". It can also negate nouns that aren't preceded by any article, like some nouns in the plural and uncountable nouns.
The latter is the case here. I copied the previous paragraph from this more extensive explanation on niet vs. geen.