A number of submarine designs have a double hull, but it's not for armor protection.
It kind of is, but not in the same way a torpedo bulge was. Most double hull designs do use the space between hulls for other things, but the exterior hull definitely protects the pressure hull.
Typical submarines have a single major pressure hull which is cylindrical in shape because a circle is the best shape for resisting pressure, however you can put down more than one pressure hull side-by-side to make a relatively fat submarine. The i-400 class had more than one pressure hull, with two on the base to provide a fat, stable runway, and another hull for the aircraft hangar up top. While much later, the Typhoon-class (AKA "Red October Submarine") had several pressure hulls.
While these aren't added on for "armor" purposes, they do in fact make the submarine harder to destroy, as each pressure hull is compartmentalized and damaging one doesn't (or at least shouldn't) flood the others. This was a sort of feature for a significant target like a Typhoon, since it meant an attacker needed to hit your prize boomer multiple times to sink it while its escorts could be fighting back. Y'know, at least until you hit the point where it starts sinking from taking on too much water, and if you're in deep seas, you're just going to drop past crush depth...
Doesn't Zuikaku Kai Ni have one of the best defenses in the game despite the lack of chest size?
It certainly does.Good morning!Let's start with measuring you all up!You don't need chest armour when you're underwater, do you!?
This feels just like a regular health checkup, doesn't it?Seems like we're going to have our three sizes measured now.So we really need to measure that?Especially the B. And also the B.The thickness of one's chest armour is directly linked to survival on the battlefield.The battlefield is so unjust...An unjust battlefield is just the way of things.Your drag'll go up, and the buoyancy'll mess up your diving, won't iiiit!?Stop unjustly venting your anger.Hyaaa! Whyyyy!?Ah♥Bustiness Gap Society