Through Waters Deep by Sarah Sundin

It is 1941 and America teeters on the brink of war. Outgoing naval officer Ensign Jim Avery escorts British convoys across the North Atlantic in a brand-new destroyer, the USS Atwood. Back on shore, Boston Navy Yard secretary Mary Stirling does her work quietly and efficiently, happy to be out of the limelight. Yet, despite her reserved nature, she never could back down from a challenge. When evidence of sabotage on the Atwood is found, Jim and Mary must work together to uncover the culprit. A bewildering maze of suspects emerges, and Mary is dismayed to find that even someone close to her is under suspicion. With the increasing pressure, Jim and Mary find that many new challenges–and dangers–await them.