In this study, we showcase a remarkably powerful strategy for enhancing the performance of bismuth vanadate photoanodes for water oxidation by combining the bulk doping of BiVO4 with molybdenum and its surface modification with a well-defined molecular cobalt polyoxometalate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) via a simple binder-free impregnation protocol. The best-performing optimized Mo-BiVO4/CoPOM photoanode exhibits a photocurrent density of 4.32 mA cm -2 at 1.23 V vs. RHE under AM 1.5G (1 sun) illumination and an applied-bias photoconversion efficiency (ABPE) of ~0.73%, which is an improvement by the factor of 24 and 2.5 compared to pristine BiVO4 and Mo-BiVO4, respectively. Notably, our detailed mechanistic investigations disentangle and elucidate the respective contributions of the Mo-doping and modification with CoPOM to the performance enhancement for water oxidation. The positive effect of Mo-doping is shown to be related to enhanced electronic conductivity and passivation of surface states. However, these beneficial effects are operative only at relatively high applied bias potentials (> 0.9 V vs. RHE), whereas at lower bias potentials (< 0.7 V vs. RHE) they are counterbalanced by strongly detrimental effects related to increased concentration of electron polaronic states induced by the Modoping. In other words, there is an inherently negative impact of Mo-doping on the maximum achievable photovoltage in Mo-doped BiVO4 photoanodes, an important aspect that has not been fully recognized and experimentally elucidated so far. The deposition of CoPOM enhances photocurrents in the whole potential range.