This is number nine in Randy Wayne White's Doc Ford series. It opens with a seeming tragedy: while four friends are scuba diving a wreck in the Gulf of Mexico, their boat sinks. One makes it to a navigational buoy and is rescued by the Coast Guard; the others are not found after an exhaustive search.
That would probably be that, aside from speculation, if one of the lost divers was not a close friend of the Dinkin's Bay crew, including Doc Ford. The surviving member saw just enough to raise a glimmer of hope in Doc's mind, and he uses his contacts and wiles to pursue the real story. The journey is tough but gripping, the climax grim.
This is (yet another) darn good yarn. Were I to quibble, I'd quibble that Mr. White seems at time to be padding things out to a contractually-agreed word count with digressions, flowery description, and a couple irrelevant subplots. But no matter, because White probably makes it more entertaining than anyone else could. You can actually learn stuff here, not just marine biology, Doc's adopted profession, but also self-defense, meteorology, South American politics, and on and on.
It seems that, in this book, Doc has achieved a breakthrough in introspection, finally making progress in integrating his past life in supersecret service to America with his current persona as a marine biologist. We'll see how this plays out.