You raise a good point here Dave. Since the stern went down after assuming a near vertical position, the pressure differential between the lowest portion and the highest would be head of about 300 ft. That's about 9 atmospheres differential pressure. And that difference would be true even after the ship went below the surface as it went straight down. But any trapped air would compress as water from the bottom moved up toward the top, and the differential would drop until equilibrium was achieved, or the top blows off. And, if my calculation is right, equilibrium would be achieved when the air compresses to about 1/3 its original volume, if nothing blows first like a hatch near the top.