Comment by bell-cot

Roughly speaking, air/liquid interfaces are excellent protection from explosions, and more are better.

The torpedo defense systems in the most advanced WWII battleships & carriers were about 5 layers thick: sea (may contain enemy mines & torpedoes) | empty compartment (air gap) | ballast/fuel tank | air gap | ballast/fuel tank | yet another air gap | important stuff (engine rooms, powder magazines, whatever)

And to be reasonably effective, each air & fluid layer needs a certain thickness - which scales roughly with the size of the explosive charge.