Direct solar water splitting is considered one of the most promising approaches for an environmental-friendly renewable and sustainable energy resource. Significantly, the capacity to directly split water is ideally suited for large-scale hydrogen fuel production. While tremendous progress has been made in photoelectrochemical (PEC) water splitting in the past decades, it is still not suitable to split pure pH neutral water efficiently without bias or sacrificial agent. On the other hand, direct photocatalytic overall water splitting still suffers from critical issues including low efficiency and poor long-term stability. In this study, we demonstrate that, through controllable dopant incorporation in Ga(In)N nanowire heterostructure, the surface charge properties can be tuned to provide the appropriate Fermi level and/or band bending. This allows the photochemical water splitting to proceed at high rate with an apparent quantum efficiency (AQE) ~20%. Furthermore, the AQE can be boosted up to ~45% in a Ga(In)N photochemical diode nanostructure by creating a p-p+ lateral junction at nanoscale, to induce unidirectional flow of photogenerated charge carriers. Consequently, a solar-to-hydrogen (STH) efficiency of ~3.3% is achieved, which is significantly higher than many of the state-of-the-art efficiencies in direct water splitting. We have further shown that, by combining the synergistic effect of water oxidation and proton reduction co-catalysts (Co3O4 and Rh/Cr2O3, respectively) and by optimizing the surface electronic properties, p-GaN/InGaN nanowires can exhibit substantially extended long-term performance-stability for >580 hours in more realistic photocatalytic conditions, that is pure water and concentrated sunlight. Such remarkable long-term stability is the longest ever measured for any semiconductor photocatalysts/photoelectrodes without protection/passivation layers in unassisted solar water splitting with STH >1%. This study further explores the effect of Sb-incorporation in Ga(In)N to selectively tune the band-edges for enhanced absorption. The Ga(In)N nanowire photocatalytic system in this study not only outperforms most of the typical photocatalysts reported so far, in the aspects of both STH efficiency and stability for unassisted overall photocatalytic or photochemical water splitting, but also provides critical insight in achieving high efficiency artificial photosynthesis, including the efficient and selective reduction of CO2 to hydrocarbon fuels.