Due to its superior electrical properties and band gap suitable to absorb visible light, WO₃ is an exceptional photoanode material for photoelectrochemical water splitting. The hydrothermal growth of WO₃ photoelectrodes is well established, based on seed layers used to grow WO₃ nanosheets directly on substrates. However, the influence of this seed layer on the photoelectrochemical performance is unknown so far. We therefore present a detailed investigation on the influence of seed layer preparation on the photoelectrochemical water oxidation performance of hydrothermally-grown WO₃ photoanodes. Those thin films were carefully characterized by X-ray diffraction, electron microscopy, Raman spectroscopy, absorption spectroscopy, and multiple electrochemical and photoelectrochemical methods. A high spinning speed or long spinning duration for spin coating leads to a low seed layer density, which benefits the formation of m-WO₃ resulting in an increased performance. The combination of high absorption, high donor density, low onset potential and high charge carrier injection efficiency leads to the highest photocurrents. For a phase pure m-WO₃ photoanode with a film thickness of 2.3 μm and a flat band potential of 0.39 V_RHE, a photocurrent density of 1.51 mA cm⁻², an IPCE of 60% and high separation efficiency of 34% was observed without any co-catalyst.