From: Winnipeg, MB
Ok, now I understand your amazement.
I-boats are very tough, almost a tough as US fleet boats [not S-class, which are smaller]. It isn't so much the number of hits as the quality of hits that matters - most hits are "minor" hits [near miss causes damage] that cause some system damage and minor leaks or minor fires, but much of that can be repaired at sea. That I-boat probably got all minor hits, and usuall after 25 or so minor hits the sub gets forced to the surface and is riddled with gunfire hits from .303 machine gun size to the biggest guns in the TF. It is rare for guns over 5" to score hits though.
However a major, penetrating hit by an ASW weapon will cause flooding and severe system damage, along with the possibility of fire. Three major hits will put an I-boat in danger of sinking, with flooding around 70% and likely to increase if minor hits are scored afterward. Severe system damage also hampers subsequent damage control and increases the chance the sub will sink.
RO-boats are a smaller and a bit less robust than I-boats. The SSX mini-subs are eggshells, damaged by near misses that would not be close enough to damage a full sized sub, and doomed by any major hit. Remember, the mini-subs were not designed to dive deep but to penetrate harbours so their hulls are not the same tough steel as full sized subs.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth