This gadget costs more than the camera; go figure.

Aquatica Launches an Underwater Housing for the Canon 5D Mk III

The Canon 5D Mk III is a very good camera, and it's  well suited as an underwater camera.  If you must get some shots of aquatic life, the new Aquatica A5D Mk III allows you to do that. At a price, of course.

The casing costs $3,199 and is designed to be number one in it's class.  The housing was made to be compact and lightweight, but Aquatica it's  strength  not a weakness.  You'll get can dive to a 300ft depth with the Mk III, with the potential to upgrade to 425ft.