Shinano was not completed when she sailed. A number of “minor” details were missing, such as the gaskets around the waterproof doors. And her crew was untrained; when it came to damage control, they were probably little better than a cruise ship full of tourists.
Archerfish’s torpedoes would probably not have been fatal if Shinano was completely fitted out with a trained crew. But by this stage, Imperial Japanese Navy is one of the poorer navies in the world.
Yes, “Voyage of the Damned” for all IJN vessels still afloat...
I would think that any remaining ships that had their original pre-war officers and crews intact would be extremely competent, but running low on fuel supplies and zero air cover and submarine shielding does make them sitting ducks.
Getting regular delusional orders to “Attack, attack” from Tokyo doesn’t help, either!