Why gunfire can sink ships through the waterline

What can naval gunfire do better than bombs or torpedoes?

Shells can rapidly make flooding holes as well as damage exposed systems, which is why belt armor protects the vulnerable waterline.

Shells striking around the waterline can open numerous holes and threaten buoyancy. Belt armor is concentrated there precisely because the hull is most vulnerable to flooding in that band. Gunfire also reaches the target repeatedly and can combine underwater damage with destruction of less protected equipment above it.