Lead‐free layered hybrid double perovskites (LHDPs) promise stable, non‐toxic optoelectronic devices, but operating these materials in applications with a direct semiconductor/water interface remains a challenge. We show that a bulky, hydrophobic naphthalene diimide dication spacer (NDI‐dH 2+ ) in (NDI‐dH) 2 AgBiI 8 yields threefold‐higher in‐plane carrier mobility and an order‐of‐magnitude lower trap density than analogous aliphatic or aromatic spacers, together with exceptional tolerance to humidity and liquid water. This enables the first photoelectrochemical operation of an LHDP in aqueous electrolyte (producing 0.25 mA cm −2 at +1.0 V vs RHE under 1‐sun illumination), sustained beyond 1 h with a hole‐transporting overlayer. Notably, light harvested by the organic spacer contributes productively to photocurrent, despite a type‐I nanoheterojunction. Overall, this work establishes key routes of spacer engineering for robust lead‐free LHDPs.