The Smith family, and the few stalwarts of the Wolf Squadron, continue their campaign to reclaim the Atlantic Ocean from the zombified humans one ship at a time. So far they have pulled a few hundred people from small ships and lifeboats. Then the difficulty of rescuing those trapped on a superliner cruise ship compels what remains of the USA's government to take action. They authorize a submarine to provide necessary equipment and in the process reveal that they have been shadowing the ragtag fleet for a few weeks. The survival of a few submarines is good news, but they have an ulterior motive. Without a vaccine the thousands of crew members on those subs will die.
To Sail a Darkling Sea - more action in Black Tide, book 2