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![](https://beehaw.org/pictrs/image/3f6f8c7f-8f4e-48a6-b56f-748fb4d05eb0.png)
There are ways of creating oxygen onboard submarines. The only real limits to time submerged is the amount of food the boat can carry.
Here is a video by Destin from the Smarter Every Day youtube channel explaining oxygen generation onboard submarines. https://youtu.be/g3Ud6mHdhlQ
Should have been in front of a firing squad.