A proposed submarine substitution runs into limits of speed and cargo capacity. The cruiser could reach its destination more quickly, so secrecy alone would not settle the choice. A viewer also points out that the components would not fit in one submarine. Moving them underwater would have required multiple boats.
Notable Point · 45:10-45:35
Why a cruiser could beat submarines for an urgent cargo
Speed and cargo capacity complicate a proposal to substitute submarines for a cruiser.