When under full steam, a ship doesn’t travel in a straight line. Of course, it tries to. But the current of the water means the direction is always slightly altering. So the helmsman is keeping an eye on the destination, where he wants it to go, and adjusts constantly to make sure the lumbering ship maintains something like the desired course.
Aurelius’ ship was mostly on course. Noone suspected the presence of the submarine. Noone saw the trail of bubbles that signalled the approaching torpedo. Noone expected it. But then came the explosion. If the ship sinks or stays afloat will depend very much on what the crew does. And whether the submarine captain has fled after his perfect shot, or hangs around to deliver the coup de grace.