Why don't diesel powered submarines produce oxygen using electrolysis of water whilst submerged?

Modern DE submarines do have limited O2 generation capability, though the main reason it's limited is due to the enormous power consumption that electrolosis requires. Even on the surface with their diesel engines running, the electrical output just isn't enough for sustained O2 generation of any magnitude, and submerged battery operations negate using it. Nuclear Powered submarines generate more than enough power to produce enough oxygen to stay submerged indefinitely.