Still missing the key issue...
Lack of oxygen is not the problem, excess carbon dioxide is the problem. Releasing compressed air into the boat doesn't do any good.
Your lungs only absorb 4% of the oxygen in the air that you inhale. When you exhale, you send the "excess" oxygen back out, along with "waste" carbon dioxide that ads to that already in the air. You can live and function for a long time in an atmosphere with steadily decreasing oxygen level; as low as 15-16% has been proven to be livable. Saturation divers typically work with gas mixes that have as little as 11% O2.
But, you can't function very well at all when the carbon dioxide level rises above a mere 1%, and 2% is deadly. The type of CO2 scrubbers used today didn't exist in WWII; the ventilation system had bypass sections where canisters of CO2 absorbent could be used to control the carbon dioxide level. Of course, when running silent, you can't run the ventilation system, so simple cans of "quicklime" CO2 absorbent were available.
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