Karin implies Roshi tried to take the "magic" water illegitimately, so I don't think he was ever pure. Karin gave it to him hoping he'd be able to ride it someday.This brings-up a good question: was Muten-Roshi-sama ever able to ride it? If not, why did Karin give that to him? Was Muten-Roshi-sama ever pure then?