DHeckrotte
Guru
Actually, G, a shoebox is pretty poor in torsion until you put the top on. Shawn has it right, the deck makes the open shoebox of the hull into a complete box. It's just that the boxtop is recessed down by the height of the bulwarks. Grab the bulwarks of your boat at one side of the bulkhead door; you'll find it's quite flexible.
All that said, if your deck isn't rather firmly tied to the sides of the hull, you don't really get the structure you'd wish (and which is free for the having if the deck is firmly attached!). Lots of us have boats whose structure is a little light there; that's why the scuppers crack. My 'back porch' floor/deck is only lightly tagged to the hull at the sides and a couple places across the transom. And my scuppers are cracked.
All that said, if your deck isn't rather firmly tied to the sides of the hull, you don't really get the structure you'd wish (and which is free for the having if the deck is firmly attached!). Lots of us have boats whose structure is a little light there; that's why the scuppers crack. My 'back porch' floor/deck is only lightly tagged to the hull at the sides and a couple places across the transom. And my scuppers are cracked.