For AA 50:
"Page 30 of the rulebook,
Submersible: Anytime a submarine would otherwise roll the die to attack or defend, it can submerge instead. This
removes it from the combat—it can no longer attack or take hits in that combat. Whenever a round of combat starts and a submarine is in combat with only aircraft, it can submerge (before aircraft fire)."
Step 2. Attacking Units Fire
Submarines have the Surprise Strike ability. Both attacking and defending submarines fire before other combat units during a sea battle. (See “Surprise Strike” in Unit Profi les: Submarines, pg. 30.)"
If you link the two rules, the sub vs. sub battle would look like this.
Attacker chooses to attack a sz with a sub with a sub.
Attacking units fire but since both attacking and defending subs fire before other units do (or at same time in theory). The subs can decide to attack/defend or submerge.
Resolve either attack/defend or submerge.
If subs submerge, than the submersible ability takes effect “which removes it from combat - it can no longer attack or take hits in that combat.”
Attacking sub attacks and the defending sub sumberges, even if the attacker hits, the submersible ability allows it to survive.
For AA 1942 based on AA 50 rules but clarified.
Attacking sub decides to attack or submerge first
Defending sub decides to attack or submerge next.
But this also needs to be tied in with the submersible ability.
The reason why I asked the question originally was that for Revised, one of the attacks UK does on UK1 is to attack the Japanese sub with a UK sub.
With the AA50/AA 1942 rules, this is not possible as the Japanese sub will submerge immediately.
That’s my interpretation but I wanted to confirm with Krieghund to avoid any issues as our group was having a FTF AA 1942 game.