"This water is pure."
Translation:Cette eau est pure.
You are right "cette" is the feminine demonstrative determiner and "ce" is the masculine, although "cet" is used in front of nouns starting with a vowel. But "ce" also is a neuter subject pronoun and appears a lot with etre: "C'est un avocat." This little word "ce" gives me (english speaker) no end of trouble.
My mistake. The masculine demonstrative determiner is ce or cet (cet before masc noun leading with vowel or h); the feminine demonstrative determiner is cette (with or without leading vowel). So, the correct form in this case is 'cette eau' -- and that is now accepted as some of the comments below indicate.
The question is asking for "this water," not "THE water," and yet the correct form "cette eau" is rejected... It looks to me as if it's very much used by the native speakers: http://www.aquawal.be/fr/source-de-vie/eau-dans-le-monde/index.html
Because L'eau would mean "the water". "The" means the water has been identified, but not where it is. For instance, from a great distance away, one might say, "I have been up in the mountains and seen the streams. The water is pure." Whereas "This" implies some kind of locality: the water that one was referring to either in physical (or, less commonly, temporal) context.