Red Tide — Jeff Lindsay

From the New York Times�"bestselling author of the Dexter novels: “A game of cat and mouse that builds to an utterly terrifying climax on the high seas” (Publishers Weekly, starred review).
 
Ex-cop Billy Knight wants to ride out Key West's slow season with the occasional fishing charter and the frequent beer. But when he discovers a dead body floating in the gulf, Billy gets drawn into a deadly plot of dark superstition and profound evil.
 
Along with his gun-happy friend Nicky, and Anna, a resilient and mysterious survivor of her own horrors, Billy sets out to right the wrongs the police won't -- putting himself in mortal peril along the way.
 
“This is a thriller of the highest order.” -- Booklist (starred review)