Interfacial H2O is increasingly recognized as an active participant in electrocatalysis, yet a central challenge remains: how to deliberately and predictably program its microscopic structure to steer proton-coupled electron transfer (PCET) and selectivity. Here we introduce nonmetal doping as a materials-encoded handle to tune interfacial hydration on a model SnO2 catalyst without changing its bulk phase. Across N, P and S dopants, S uniquely stabilizes a disordered, predominantly H-down hydration motif at the catalyst-electrolyte boundary, which promotes H2O activation and directional proton delivery, accelerating PCET toward formate formation. Notably, this enhancement arises despite weaker binding of CO2-derived intermediates, revealing a solution-structure-controlled selectivity lever that can override conventional binding-energy-based expectations. By establishing a causal link between a doping-defined hydration motif and reaction kinetics, this work elevates interfacial H2O from a "context" to a designable variable, and suggests a broadly applicable strategy for optimizing PCET-governed transformations via interfacial solvation engineering.