Light-driven water splitting into O2 and H2 is a process that could one day become a viable part of an advanced energy-conversion methodology. As the reaction is rather complex, a better understanding of the mechanism(s) can be achieved using modeling and comparison between theory and experiment. One prominent approach is calculation of either of potential-energy surfaces (PESs) or even free-energy surfaces. Here, we will look at mechanisms of catalytic water oxidation through the prism of PESs using novel complexes as chemically relevant examples. To probe mechanism(s) of ruthenium-catalyzed water oxidation, we consider several aspects of the O-O bond formation as well as further O2-evolution via the so-called binuclear radical coupling pathway and via the proton coupled mono-nuclear alternative. From our modeling, the role of radicals as well as acid-base chemistry of the nucleophillic attack can be compared with the experiment-derived data. Regarding new aspects that we have recently discovered, there will be a discussion of ligand(s)-controlled pathway selectivity, base (COO)-assisted a proton coupled O-atom transfer, plausible effect(s) of axial ligands and new information about activated seven-coordinate ruthenium complexes in low and high oxidation states of ruthenium.