" the author gave no thought to the seaworthiness of the vessel, and pictured a box to keep valuables in."
The Author no doubt did not have the necessary knowledge about sea-worthiness, I read a long article by an Engineer who showed that regardless of the shape of the vessel, the mere size would put such stresses upon the support beams within, that the boat would have sunk within hours, even in a slight swell !
Your second point is pertinent too, the Story is not unrelated to the Ark of the Covenant stories.
Our old friend Leolaia also showed in one of her wonderful threads that the whole Noah Story is an Interpolation. So, even those who threw Genesis together were aware it was not in any way History, so we cannot expect the details to be accurate in any way.