It's all been passed through many organisms, many times by now. With the exceptions of very small amounts that are created through certain types of combustion and other chemical processes. Even then the hydrogen and oxygen has likely been passed through, or been a part of, an organism.
Water gains and loses impurities along the way. Molecule by molecule, most of those impurities stay behind in the evaporation part of the water cycle. Then water falls and gains impurities anew. Some of those impurities remain in the soil as water seeps down, in the case of ground water, or accumulate as water flows over the earth. The impurities, and the potential for ill health effects, is the reason water treatment has been the norm for over a century.
Actually, it appears the amount of water does change. There may be a steady incoming trickle from space in the form of very small comet-like dirty iceballs. Water from these objects joins the water cycle. http://www.windows2universe.org/headline_universe/snowball.html