Krieghund, I think I sunk you.
On the matter of attacking subs and defending ships/fighters.
Pg. 29 - Additionally, your aircraft may attack enemy submarines.
This says nothing about defense.
Pg. 30 - This special attack only happens once at the
start of the attacking units fire step. Then, the combat sequence continues normally.
This says to me, the sub rules are somewhat comparable to the previous versions. A sub gets a special attack. A sub may submerge if opting not to attack or defend. Subs may be ATTACKED when there is a Destroyer to find them. Ships/Fighters may DEFEND against a sub attack as usual. A sub is not an invincible unit unless there is a destroyer!
If this is not understood to be the accurate assesment of the rules, please consider, the alternative is making a pretty good unit (as intended), the sub, into a dominating unit (not intended) and the whole game kinda silly.
To think otherwise, is like saying, “Oh my gosh, they built one sub, my entire fleet is dead unless I buy a destroyer?” Or “What was that? A torpedo! Good thing it missed. Uh-oh, here comes another one. What do we do?”