There are a group of hobbyists who put their lives on the line every time they perform: deep water divers. They dive to such extremes that they often lose the ability to be rational, resulting in a great number of fatalities. Shadow Divers is about the guys who found a U-Boat in American waters and their quest in finding out how it got there.
The book is fantastic. The story is so good it could pass for fiction. The reader is not only entertained, but learns about diving, the war, U-Boats, history, battle tactics and more. I highly recommend it.