AOB is simply the angle between straight ahead on the ship, and the direction to your boat; so the easiest way to work it out is to imagine yourself stood on the bridge of the ship you intend to shoot at - however far you would have to turn your head in order to see the submarine would be the AOB (i.e. if you had to look directly to the left when on the ship to see your submarine, the AOB would be 90 degrees port).
Golden Rivet put a simple and very helpful tutorial on this forum a couple of days ago on how to calculate AOB.
Chock