Deep shadow / Randy Wayne White.

Enhanced descriptions from Syndetics:

Doc Ford wrestles more than one kind of demon, in the stunning novel from the New York Times -bestselling author.

Many dangers lurk in the deep-the worst of them are human.

Thirty minutes into what should have been an easy, beginner-level dive in a remote Florida lake, the rim of a cave collapses, trapping two of Doc Ford's friends. Ford himself manages to escape and quickly surfaces to find help-but that's when his troubles only begin.

Two men are waiting for him on the shore, and they are not the kind of men you want to meet at any time. Murderers and ex-cons, they're intent on diving to the bottom of the very deep lake and uncovering the remains of a legendary plane wreck there, supposedly loaded with Cuban treasury gold. Ford's ex­pertise is just what they need. And if he doesn't want to help? He can die. His friends? They can die, too. In fact, they can die right now. . . .

As the hours tick away, two mortal struggles unfold simulta­neously, one above and one below. Neither outcome is certain, no man is safe . . . and in the deep shadow, only death awaits.