The Letter Of Marque — Walter A. Turner

Because of partially disabling injuries previously suffered, their boss at Southern Air Transport arranges for them to keep flying, shuttling passengers from St. Barts to his new undersea resort, Atlantis.

After a friend of Fancy's is murdered in a particularly horrific way and people begin to mysteriously disappear from their yachts, Fancy and Bailey are recruited by the Office of Budget and Plans, an intelligence gathering arm of the United States Navy.

They are tasked with "just keeping their eyes open" under the authority of a Letter of Marque. A chain of events draws them into a vortex of violence and intrigue.