A recent column on "The Straight Dope" website discussed the idea that water molecules are largely cycled around through the water cycle, and that therefore the water molecules that come out of your tap could have been drunk (and excreted) by the dinosaurs. Cecil Adams (the author) points out a few flaws with this argument, namely that we've been burning fossil fuels for a couple of centuries and releasing water into the atmosphere, and that liquid water is constantly dissociating and reforming; so it's not strictly true to say that every water molecule on Earth has been around since the time of the dinosaurs.
One thing that occurred to me in reading this article is that biological processes have also been consuming and producing water for many millions of years now; this would be another mechanism by which water molecules can be "reshuffled". Is is possible to estimate how much water participates in these processes (per annum)? I know that there are estimates out there about how much carbon dioxide is produced/consumed by the biosphere, for example, but I'm not sure whether one can straightforwardly extrapolate from "carbon consumed" to "water consumed," and I don't know if there are other significant metabolic processes that consume/produce water molecules.