III-nitride nanowires (NWs) have been recognized as efficient photoelectrochemical (PEC) devices due to their large surface-to-volume ratio, tunable bandgap, and chemical stability. Doping engineering can help to enhance the PEC performance further. Therefore, addressing the effects of Si and Mg doping on the III-nitride NW photoelectrodes is of great interest. In this study, doping levels of NWs were tuned by the dopant effusion cell temperature of the molecular beam epitaxy (MBE) growth. The successful doping of the III-nitride NWs was confirmed using photoluminescence (PL), Raman spectroscopy, and open circuit potential (OCP) measurements. The ionized dopant concentrations of Si-doped InGaN/GaN NWs were systematically quantified by electrochemical impedance studies (EIS). Due to the three dimensional surfaces of NWs, modified Mott-Schottky formulas were induced to improve the accuracy of ionized dopant concentrations. The highest dopant concentration of Si-doped InGaN NWs can reach 2.1x1018 cm-3 at Tsi = 1120 oC. Accordingly, the estimated band edge potentials of the tested NWs straddled the redox potential of water splitting. The PEC performance of these devices was investigated by linear scan voltammetry (LSV), chronoamperometry tests, and gas evolution measurements. The results were consistent with the quantified dopant concentrations. The current density of n-InGaN NWs doped at TSi = 1120 oC was nine times higher than the undoped NWs. Additionally, the doped NWs exhibited stoichiometric hydrogen and oxygen evolution. By doping Mg into InGaN and GaN segments separately, the p-InGaN/p-GaN NWs demonstrated improved PEC performance, compared with undoped-InGaN/p-GaN and n-InGaN/n-GaN NWs. The p-InGaN/p-GaN NWs exhibited a highly stable current density at ~-9.4 mA/cm2 for over ten hours with steady gas evolution rates (~107 μmol/cm2/hr for H2) at near a stoichiometric ratio (H2: O2~ 1.8:1). This study demonstrated that optimizing the doping level and appropriate band engineering of III-nitride NWs is crucial for enhancing their PEC water splitting performance.