Aut cum scuto aut in scuto
The worst fears for a submariner are a fire in the battery compartment, which runs out of control (AS-31), or a hot run torpedo (Kursk), though failure of a main seawater valve, and its alleged fail safe backup, is not a sight a submariner likes to see in the engineering spaces.
The primary thought in every submariner's mind is maintaining a safe submarine ratio of one surfacing to one submergence.
Analogous to the aviation maxim: Takeoffs are optional, landings are mandatory.
There was a lot to be said for not installing screen doors....