The Margo is a yacht. Its crew sees The Black Hand floating in the water after his underwater battle with Captain Flag. After he is recovered, The Black Hand murders the captain of The Margo for discovering his identity. A massive storm rises, killing everyone on board who didn't fall overboard except The Black Hand.
The villain tosses the corpses into the sea. He then sails the limping vessel to port where he has it refurbished. He then hires Black Hand's cutthroats to man the vessel.
The Black Hand becomes a high seas pirate. He and his crew board cargo vessels and murder all the hands. They take the cargo and burn the ships.
Captain Flag discovers The Black Hand's ocean borne criminality. He finds employment on a ship, hoping it will be attacked by the pirates. His plan works out, and triumphs over the pirate captain. The Black Hand is found guilty of piracy aboard the ship and executed by hanging.