The water hydrogen-bonded network is strongly perturbed in the first layers in contact with the semiconductor surface. Even though this aspect influences the outer-sphere electron transfer, it was not recognized that it is a crucial factor impacting the solar-driven water-splitting performances. To fill this gap, we have selected two TiO2 anatase samples (with and without B-doping), and by extensive experimental and computational investigations, we have demonstrated that the remarkable 5-fold increase in water-splitting photoactivity of the B-doped sample cannot be ascribed to effects typically associated to enhanced photocatalytic properties, such as band gap, heterojunctions, crystal facets, and other aspects. Studying these samples by combining FTIR measurements under controlled humidity with first-principles simulations sheds light on the role and nature of the first-layer water structure in contact with the photocatalyst surfaces. It turns out that the doping hampers the percolation of tetrahedrally coordinated water molecules while enhancing the population of topological H-bond defects forming approximately linear H-bonded chains. This work unveils how doping the semiconductor surface affects the local electric field, determining the water splitting rate by influencing the H-bond topologies in the first water layers. This evidence opens new prospects for designing efficient photocatalysts for water splitting.