the flat bottom of an will not offer protection against under-hull- detonation. on the contrary, this will trap the shock wave underneath. also, in the way of strenght, an box is always weaker then an curved surface. I can understand an waterline section for the sake of armour for an battleship, but I cannot understand why the cross section of the ship would also be boxed and flat. maybe easier to produce, but it would need a lot more material to strenghten it and by that will archieve the contrary (as welding and placing parts takes more time then steel cutting and bending, especially in these days.)
Alright, you got me.. touché.