[Iron Budokan]

It sometimes depends on what ballast or cargo the ships are carrying. If they're carring iron ore they'll go down if you dent their hull just a bit. If they're carrying balsa wood they'll tend to stay afloat even if you crack 'em in half. Okay, it's not quite like that, but unless I'm mistaken the cargos are modeled in GWX and that's why some ships might go down easier.

Of course other factors are also taken into account: where you hit the ship, what type of torp, etc.