The crew of submarines vessels produce the oxygen they need inside the submarine through a process known as electrolysis. During the electrolysis process, the hydrogen and oxygen in water is separated by passing a direct current to water. Since water is readily available in the ocean, and submarines have enough electricity stored in its batteries, oxygen is not really a problem.

When i was young, i loved watching James Bond films and his submarine adventures. I kept wondering how life without oxygen was possible in submarines, was not able to get appropriate answer untill i lost that thought.
Not untill now, i never knew the submarine vessels produced the needed oxygen through electrolysis
Thanks for sharing this information