Short answers, pulled from the story.
The United States adopted an official policy of unrestricted submarine warfare on the 7th of December 1941. U.S. Navy Chief of Staff issued a directive to Pacific commanders six hours after the Japanese attack on Pearl Harbor authorizing all submarines in the region to execute air and submarine warfare without warning.
Early U.S. torpedoes failed due to four major engineering faults in the Mark 14 model that were mass produced without adequate testing. From December 1941 to late 1943, these weapons achieved only a 20% success rate because they often failed to explode despite scoring direct hits.
Allied submarines sank around 1,300 Japanese merchant ships by the end of the war in August 1945. This destruction reduced the Japanese merchant marine to less than a quarter of its tonnage from December 1941 when capacity totaled 7 million tons.
The battleship Musashi remains the only Japanese battleship lost to a submarine during the conflict. It was sunk alongside other heavy units like fleet carriers Shōkaku and Shinano which was the largest vessel ever sunk by a submarine.
American submarines rescued 504 airmen including George H. W. Bush who later became the 41st President of the United States. These rescues began in early 1944 when U.S. submarines started saving crews of aircraft forced down over the ocean.