Sea Castle: A Thriller - Andrew Mayne

A Wall Street Journal bestselling series. A deep diving investigator is pulled into the depths of a string of unsolved serial murders in a riveting thriller by the Amazon Charts bestselling author of Sea Storm. When a young woman washes ashore on a Fort Lauderdale beach, Sloan McPherson of the Underwater Investigation Unit is called in to consult. Sloan's instinct says murder, but even then, there are too many questions.
For answers she reaches out to Gwen Wylder. The Miami homicide detective is notorious for being manipulative, bitter, a tyrant to her peers, and wicked smart. And she demands something in return from Sloan: fresh insight into seemingly unrelated cold-case murders and disappearances-and a possible serial killer trolling the Florida coast.
As loose ends of the old files begin to come together, another woman disappears. Sloan and Gwen are certain she's the newest link in a deadly chain. They are determined to track her down before she dies, but they soon find themselves in uncharted waters. And the deeper Sloan and Gwen go, the stranger the case gets.