Blood Tide — J.D. Cameron

The ambition and folly that destroyed three-quarters of humanity lives on in the radioactive rubble of nuclear holocaust. In the South Pacific, an insane fanatic and his army of bloodthirsty killers have begun obliterating the last scattered outposts of civilization. The madman's goal: to rule what remains of the shattered Earth. For that, he needs the awesome power of the U.S.S. Liberator...and he holds captive the one man who can make his demented dream a reality. For unless the crew surrenders the mighty submarine intact...Captain Tom Donovan will die!