Naval battles get really complicated when both subs and destroyers are present. You are best off to treat each sub-step (double meaning), separately, sorting out OOL at each juncture.

For each round where subs are present, conduct the opening fire sequence, sort out the consequences and then conduct the remainder of the battle.

Here's the rules - all three should be read in conjunction with each other:

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Submarines
All attacking and defending submarines present on the battle board fire in this step regardless of whether either side has destroyers present. If submarines are present on both sides, they are considered to fire simultaneously. Roll for attacking submarines first. The defender moves any units hit by the attacker's subs to the casualty zone, then defending submarines fire. If the defender scores any hits, the attacker moves his hit units to the casualty zone. Submarines can fire only on sea units.

AND

Step 3: Remove Opening Fire Casualties
Clear both casualty zones, destroying all units there. (Return them to their owners’ inventories.) ....

AND

Submarine Casualties and Destroyers
If a destroyer is present on the battle board in a combat involving enemy submarines, the submarines' casualties can fire back at the normal time in the combat sequence, provided they have not already fired in step 2. They remain on the battle board in the casualty zone until Step 6. The  destroyers provided enough advance warning to allow return fire.

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Note that REGARDLESS of whether there are destroyers present, both sets of subs get to fire in the opening round. The presence of the destroyers does NOT negate this opening fire privilege. What it DOES mean is that if I have a destroyer on my team, any of MY units hit by any opposing subs will still get a chance to take a shot when "normal" firing begins, but they'll be shooting from the casualty zone on the battle board.

I hope this helps...
BW, JAG