Free elemental hydrogen isn't common. Most are stuck as water, or inside hydrocarbons in the first place (see natural gas), and using those sources basically defeats the purpose.

So therefore, it would take more energy to free that hydrogen from water than the recovered output would give you - which is fine, as oil is a dense storage of energy, and current technology is geared towards using it. However, it makes more sense to directly utilize the electricity that is expended freeing hydrogen in the first place.