In Quantum Field Theory, the concept of "the same water molecule" is not a well-defined concept, because of wave-function symmetry. (The only water molecules that are fully distinguishable are those where the isotopes of hydrogen or oxygen are different.)
You can still chart the history of a single molecule in theory, you just can't tag two molecules and tell them apart later without context. I guess that's another way of saying "the only true quantum configuration is the entire universe" though.