@Krieghund:
The decision of whether or not to submerge is made both sides before any shots are fired, with the attacker deciding first, so a sub can only attack another sub if the defender chooses to fight (or if there’s an attacking destroyer). This will be in the FAQ.
That’s not the way the rule is written. According to the manual, page 30
“Anytime a submarine would otherwise roll the die to attack or defend, it can submerge instead.”
Thus a sub attacked by another sub cannot submerge until after fired upon. A defending sub doesn’t roll until after the attacker does. Unless the manual is flat wrong. The manual mentions an exception for aircraft, but not enemy subs.