We report the thermal synthesis of a heterogeneous Co–phosphine-based water oxidation catalyst that can be easily coated onto a variety of substrates. The system is active in neutral water and performs best with borate as electrolyte, in which the onset of catalytic current occurs at 1.51 V vs. RHE, with a Tafel slope of 68 mV per decade. Stable water oxidation is observed for over 40 hours; including in seawater with borate electrolyte, in which the catalyst is selective for water oxidation over chloride (85% Faradaic efficiency). The organic portion of the catalyst appears to play a crucial role in catalytic activity.