The trivacant triruthenium-substituted polyoxometalate α-K₆Na[{Ru₃O₃(H₂O)Cl₂}(SiW₉O₃₄)]·17H₂O (1) and two sandwich-type Co- and Ni-containing silicotungstates K₁₁Na₁[M₄(H₂O)₂(SiW₉O₃₄)₂]·nH₂O (M = Co²⁺, Ni²⁺) (2, 3) based on the same Keggin-type building block were obtained through variation of a solution-based protocol. All compounds were investigated with respect to their performance as visible-light-driven water oxidation catalysts (WOCs). Analytical characterizations and photocatalytic reactivity of (1) are correlated with results of B97D/Def2-TZVP computation. The WOC activity of (1) and (2) was found to arise from their immobilization into a recyclable POM–photosensitizer complex (POM–PS). These POM–PS complexes are formed in the initial phase of the reaction, and they prevent POM decomposition into colloidal oxide catalysts.