Honestly on a surface ship I can't think of very many 1-hit-kill components. Even in WWII tiny little Destroyer Escorts were able to withstand multiple shell hits from Japanese Heavy Cruisers and even the Yamato. (The Battle off Samar, if you want to wiki it).

With the move toward computerization and long-range missile-based combat there's probably a lot of risk with the Fire Control System, Radars (e.g. AEGIS), stuff like that. But even blind you can at least run away, and the CIWS has an independent fire-control radar for last-resort self-defense.

Submarines are more problematic. There's only the one pressure hull, only the one reactor, only the one main propulsion train, and watertight compartmentalization only exists for the reactor compartment.

This makes everything about subs more expensive since all work that affects these things has to be formally controlled and QA'ed, re-tested, etc. to avoid losing more subs like we lost Thresher and Scorpion.