This is the reaction that occurs when hydrogen combusts:
2H2+O2⟶2H2O2HX2+OX2⟶2HX2OSimilarly, this is the combustion reaction for methane, a representative fuel:
CH4+2O2⟶CO2+2H2OCHX4+2OX2⟶COX2+2HX2OWater is a product in both of these reactions. Thus, water represents something that has already been oxidized by oxygen, and as such there is little further energetic gain to be made by trying to react it again with oxygen