Thunder Point:: by Jack HIggins.

An adventure and mystery and suspense thriller attractive for the background and the story both, in proportion depending on the reader.

A deep sea diver in tropical western Atlantic discovers something extraordinary on a dive when a current takes him to a place he has never been, and no one else has in a few decades from the look of it, where on a deep sea ledge he discovers a submarine wedged precariously - but it is a very valuable discovery, it is an old submarine, German in fact from the last war. On investigation there is more discovery, more revelations. But anyone doing any further investigations is in peril, since not only one might be trapped in the submarine, the submarine itself is in a very precarious position and might dislodge and go to the bottom of the ocean from the sort of deep sea hill it is resting in a nook of. Yet there are secrets of the submarine, what it was doing there, who was on it, and so forth, not to be dismissed easily.