Whei ho,
Shouldn't AP Bombs, which overpenetrate, cause floodings? I mean, look at it from a scientific angle; A Bomb, crashing through deck and hulk ending up beneath the ship in the ocean, causes a hole in the hulk, right? And a hole below the waterline causes a flooding, or am i wrong? I mean, i certainly never have seen a ship with an unwanted hole in it's hulk that has no issues with water coming inside, even when the ship has bulkheads in order to minimalize consequences of floodings.
Same ofc holds for AP/Rockets shells that overpen and exit the ship and exit below the waterline, depending the shells ballistic arch.
And from my pov it's just a worthy equivalent to HE's chance to cause a fire. (yeah, yeah, AP is more likely to hit a citadell, bla,bla,bla, but AP is also more likely to riccochet off or shatter. And btw you can citadel with HE as well.)