As has been stated 100% is lethal, .... 'curtains for certain'. Depending on class & maybe number of crew(?) - a rough rule of thumb I use is approx 48 hours submerged for a fleet-boat before I need to surface and 'air the boat'.
Happy Hunting!
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" Bless those who serve beneath the deep,
Through lonely hours their vigil keep.
May peace their mission ever be,
Protect each one we ask of thee.
Bless those at home who wait and pray,
For their return by night and day."
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