In the August 18, 2008 New York Times there is an article with a slide show of photographs about the US Navy aircraft carrier, Oriskany. This vesssel was sunk off of Pensacloa Florida as a part of the Navy's reef program.
The superstructure of the ship is in water shallow enough for recreational divers using compressed air but deeper portions of the vessel are dangerous for sport divers and can pose a risk for decompression sickness in the untrained non-technical diver.
The article can be accessed at this web site:
http://www.nytimes.com/2008/08/19/science/earth/19ship.html?ref=science