Variable, I understood the defending submarines to only be allowed to make this attack under the “Surprise Strike” rule. This rule, written on page 16 of the rule book reads as follows:
Each attacking submarine conducting a Surprise Strike attack rolls one die. Attacking submarines that roll a “2” or less score a hit. After the attacking player has rolled for all attacking submarines, the defender chooses one sea unit (submarines cannot hit air units) for each hit scored and moves it behind the casualty strip. (Note that undamaged capital ships that are hit only once are not removed.)
Then each defending submarine conducting a Surprise Strike attack rolls one die. Defending submarines that roll a “1” score a hit. After the defending player has rolled for all defending submarines, the attacker chooses one sea unit for each hit scored and removes it from play. (Note, undamaged capital ships that are hit only once are not removed.)
Since the subs I spoke of are not on the attacker’s side (the side making active moves), I believe they are rolling for 1s and not 2s. This is why I posted the scenario in order to begin working through the many possible misconceptions.