We describe a simple method for measuring the 'differential pathlength' of photons in a scattering medium utilizing the spectral absorption features of water. Determination of this differential pathlength is a prerequisite for quantifying chromophore concentration changes measured by near-infrared spectroscopy (NIRS), and the method proposed here achieves this without the need for time- or frequency-resolved measurements. The quantification of tissue chromophore concentration measurements is a major goal in NIRS research, allowing, for example, the non-invasive measurement of blood flow and volume in the brain and other organs. We present the results of validation experiments performed on tissue phantoms comparing the differential path estimates yielded by water absorption and time-resolved measurements, finding that the weak water absorption feature at 820 nm can yield a differential path estimate in addition to the main feature at 975 nm. We also present results from in vivo studies in which we find that whilst the 820 nm feature is measured with lower accuracy for a given light flux than the 975 nm feature, it is intrinsically a more accurate differential path estimator. Studies on the adult forearm showing differences between time-resolved and water absorption differential path estimates suggest that the measurement of both could aid in quantifying NIRS signals in heterogeneous tissues.