Here, it is "rent" because "vand" is neuter, or a t-word, and indefinite, so the adjective needs to agree (with the "-t" suffix). If it was something that wasn't neuter (such as "en bil") then it would be "en ren bil". If it were definite, or plural, then you would use "rene" ("rene biler" or "det rene vand")
Most people I know would say this differently. Rather than "behøver" they would say "skal bruge", as in "Pigen skal bruge rent vand". The meaning is identical, but the "behøver"-version sounds foreign to me. I mostly see the "behøver" version in text that has been translated from english.