I think drydock/leave is such an essential feature to add.
With this added feature, you could add fatigue to the crew iver time on patrol. There was a reason these salts were granted a few weeks leave! Sub battle fatigue is extremely hard physically and mentally. Additionally, the sub suffers as well and eventually needs to be drydocked! This could be randomized based on time at sea and damage. This way it feels much more immersive and time actually passes after a long patrol. All of these could be optional features!
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