I too have been searching for answers but in a simple sub vs. sub context. Manual, pg 18,“Step 2, General Combat…both attacking and defending submarines can choose to either make a Surprise Strike or Submerge…”,pg 30,“Submarines, Special Abilities, Submersible:…The decision is made before any dice are rolled by either side (the attacker decides first) and takes effect immediately.”, this is reasserted on pg 4 of the March 2009 FAQ, “Submarines Q. Exactly when is the decision made whether or not to submerge submarines? A. The decision whether to submerge submarines is made before any dice are rolled by either side in an exchange of fire , and the subs submerge immediately. The attacker decides before the defender.”. “Decides” does not equate to “attacks”.
BGG Axis and Allies Anniversary Edition, Forum, Rules, Sept, 2009, “Can a submarine ever be attacked without a destroyer present because of the ability to submerge first”. Kevin Chapman ( Patron) answers,"…so a sub can only attack another sub if the defender chooses to fight (or there is an attacking destroyer)".
BGG (as above), in " subs vs other naval units", has other useful comments by Kevin Chapman and Don Moody on this subject.
I like the imagery of this implementation. A lone sub,or two causing isolated mayhem before sinking into the depths or just driving for their pens, until, at the slightest indication of an attack,“Dive, dive dive”. What is wrong with that…until a destroyer turns them from buccaneers of the sea into desperate survivors hoping only to escape!.