Scientists estimate that there are about 10 zillion (10x10^16) fish in the sea, which is more than 1 million fish for every human. About 95% of these fish live deeper than 500 meters under the sea. All of these fish require oxygen to breathe. They do not split water molecules into Hydrogen and Oxygen; rather they breathe the oxygen from air that is dissolved in the water. Separation of dissolved air from water is simpler, and requires much less energy, than splitting water molecules (which is the way that nuclear submarines supply oxygen to their crews). All that is needed to release the dissolved air is to lower the pressure on a body of water ('Henry's Law'). An example of this is the fizz that escapes from carbonated drinks when opening the cap.
I have looked at: 1) the amount of dissolved oxygen that exists in seawater, 2) the human oxygen consumption rate, and 3) the amount of energy required to release this dissolved oxygenin sufficient amount, and have found that it is completely feasible for a non-nuclear naval submarine to supply the needs of oxygen for the crew by extracting the oxygen from the water.
This method can replace the current air supply system for the crew (storage in cylinders) with the obvious advantage of much longer, almost limitless, underwater capacity.
Applicable for: Large and small non-nuclear submarine, SDVs. individual divers, underwater habitats, oil rigs and more.
Status: patents granted in the US and in Europe. Prototypes developed.