If you have a sub and the enemy has a sub and a battleship, you can make a Surprise Strike, but the defender can take the hit with whichever unit he wants (and if it's the sub, it would still get return fire if your opponent also chose to do a Surprise Strike). Having a Destroyer prevents your opponent from making a Surprise Strike, not yourself.
Step 2. Submarine Surprise Strike or Submerge (Sea Battles Only)
This step is specific to attacking and defending submarines.
Before the general sea battle takes place (steps 3–5), both attacking and defending submarines choose to either make a
Surprise Strike die roll or submerge. However, if your opponent has a destroyer in the battle, your attacking or defending
submarines cannot submerge or make a Surprise Strike. Combat proceeds normally, and your submarines fire along with your
other units in step 3 or 4.
Submerge: Attacking or defending submarines that choose to submerge are immediately removed from the battle strip and
returned to the game board in the contested sea zone, removing them from the rest of that sea battle.
Players on both sides have to decide whether attacking and defending submarines will fire or submerge before rolling any dice. The attacking player decides first.
Surprise Strike: Each attacking submarine conducting a Surprise Strike rolls one die, scoring a hit on a roll of “2” or less.
After the attacker has rolled for all attacking submarines, the defender chooses one sea unit for each hit scored and moves it
behind the casualty strip. (Submarines cannot hit air units.) Then each defending submarine conducting a Surprise Strike rolls
one die, scoring a hit on a roll of “1.” After the defender has rolled for all defending submarines, the attacker chooses one sea
unit for each hit scored and removes it from play.
Once all Surprise Strike rolls have been made, remove the defender’s casualties. This step is over for this round of
combat. Repeat this step during each round of combat as long as there are attacking and/or defending submarines and no
opposing destroyers. Any hits made during this step that do not destroy units (such as battleships) remain in effect until the
end of the combat.
Note: In both cases, attacking or defending, transports can be chosen as casualties only if there are no other eligible units.
You cannot choose submerged submarines as casualties since they have been removed from the battle.