A carbon-based gas diffusion layer comprising a fibrous macroporous bottom layer and a smooth macroporous top layer has been selected as the SVG (Fig. , Supplementary Fig. ). The material provides broadband light absorption, while its porous structure gives it the ability to float on water (density ≈ 0.49 g cm ) and allows for efficient water transport (see Supplementary Discussion 1). Additionally, a bi-layer construction and a macroscopic porous structure of the SVG provides a natural salt rejection pathway preventing salt fouling issues. The floating SVG-PC sheet was fabricated by drop-casting RhCrOx-Al:SrTiO3 powder dispersed in isopropanol onto the hydrophobically-treated SVG substrate (silanization), followed by drop-casting Nafion (Supplementary Fig. ). A Nafion® ionomer was used to better adhere the photocatalyst to the SVG and improve the interaction between the photocatalyst and the water vapour generated by the SVG, thus increasing the robustness and efficiency of our devices. The weight and density of the as-prepared sheets (consisting of SVG, PC powder and optical adhesive) was approximately 37.4±0.1 mg cm -2 and 0.67±0.01 g cm , respectively, allowing it to intrinsically float on the water surface without additional support. SEM-EDX elemental mapping confirms the presence of RhCrOx-Al:SrTiO3 on the SVG substrate (Supplementary Fig. ). The resulting sheets were studied in a top-down reactor under AM 1.5G (100 mW cm -2 ) irradiation for 22 h (Fig. ) with H2 and O2 quantification by gas chromatography (GC) and a fluorescence oxygen sensor probe, respectively. Subsequently, the vapour generation rates were measured in a custom 1.51.55.0 cm reactor with an open top (see Method section).