With the capability of localizing optical energy via surface plasmon resonance (SPR), plasmonic Au nanostructures hold great promise for enhancing the solar water splitting of semiconductor photocatalysts. While the content of Au plays a critical role in mediating interfacial charge transfer, its quantitative influence on the efficiency of plasmon-assisted water splitting is still not fully understood. This work aimed to explore the correlations among plasmonic metal content, SPR-mediated charge transfer and electromagnetic response, and the resultant photoactivity enhancement toward photoelectrochemical (PEC) water splitting. The PEC system was constructed by employing Au particle-decorated ZnO nanocrystals (ZnO-Au) as the plasmonic photoanode. Time-resolved photoluminescence spectroscopy and finite-difference time-domain simulations were utilized to evaluate the optimal Au content which attained effective charge separation and imposed a significant SPR effect for achieving the largest photoactivity enhancement. The charge transfer at the photoanode/electrolyte interface and its dependence on the Au content were examined with electrochemical impedance analysis, which manifested the effectiveness of the optimal Au content in facilitating the hole transfer kinetics. The present study reports a technical advance in the realization of the quantitative effect of Au for designing sophisticated plasmonic PEC systems that enabled efficient solar-to-fuel energy conversion.