Hobbes is correct. The destroyer must be in the battle, not just in the sea zone.
Submarine Sneak Attack
Lets say there is a british submarine against a Jap AC with 2 fighters on it.
If the submarine attacks and successfully sneaks, are all the enemy units destroyed? Or only the AC gets destroyed?
Fighters are always defending in the air, so only the carrier is destroyed. However, if the fighters cannot find a place to land within one space, they are also lost.
That means to say, the person with the fighters can move 1 space even when its not his turn?
Yes, that’s correct. The move is done after the attacker finishes all battles, but before the attacker makes any noncombat movements.