Depends on if the Sub is attacking or defending…
If the Sub was attacked by the carrier(AC) and cruiser. It has the option to submerge. If it doesn’t submerge, then the Sub would Surprise Strike at 1(since it’s defending), at the beginning of the round. If it hits, then the attacker has the option to apply the hit to his AC or cruiser. He would most likely apply the hit to his AC, since AC’s don’t have an attack value. Then, he would roll for his cruiser. If he hit, your sub is dead. If he misses, then you have the option to press the attack. Then, every round you roll for your Surprise strike first. If you hit, in 2nd or subsequent rounds, while your Sub is still alive, then the cruiser would die before being able to roll a defending dice roll.
Remember, when destroyers aren’t present, then both attacking and defending Subs fire their 1st strike before any other units.
If the Sub was attacking, then the Sub rolls the 1st strike @ 2(since he’s attacking). If he hits, same thing as above. Your opponent decides where to apply this hit. If he applies to the damaged AC or cruiser, then that unit doesn’t get a return fire, as the 1st strike killed it before it had a chance to return fire. Then, as long as the Sub is still alive it gets a 1st strike. If it hits, then that unit won’t get a return fire either, as it died before it had a chance to return fire.
Hope this helps…