Nuaticam is the gold standard in waterproof underwater camera housings and they came out with a gorgeous unit for the Sony A7 series cameras called the NA-A7. It's not exactly cheap @ ~$2850 via Bluewaterphotostore, and Sammy's Camera. However, you get what you pay for, and considering the cost of the camera and potential glass mounted it makes sense for the really hardcore divers. If you are only going in a pool or something, then consider the well reviewed much more affordable Meikon underwater housing I just posted about here >>

Nauticam NA-A7 Underwater Housing

Nauticam NA-A7 Key Features Breakdown Courtesy of BluePhotoCameraStore:

Signature Nauticam Port Locking System
The iconic red locking lever is right at home on the NA-A7... lens/port changes made quick, easy and secure.
SLR Styling with Handles Built In:
The housing is designed to be used with the shooter's hands on the handles, ergonomically placing the camera controls at just the right spot for easy use. No extra tray/handles to buy!
Laser cut stainless steel brackets reinforce the handles, making the rig sturdy and stable even with large strobes or video lights. Mounting balls are included, ready to attach strobes, video lights, and more.
Vacuum System:
The integrated vacuum check and leak detection system adds extra assurance of watertight integrity. Use it without an optional vacuum valve and it serves as a audible/visual water ingress detector. Add the vacuum valve, and get watertight integrity indication via a color coded LED light.
LCD or EVF - Your Choice
The patented switch means either the LCD or EVF can be used and the shooter can instantly switch between them - underwater!
EVF Viewfinder Options:
For shooters wanting to take the ultimate advantage of the excellent OLED EVF in the A7, the renowned 180º Enhanced Viewfinder or the excellent 45º Enhanced Viewfinder are available. It takes less than a minute to change these viewfinders and no tools are required.
Fingertip Control:
Fingertip control for all key shooting controls, including both shutter release, focus lock, both command dials, and video start/stop. All of the controls are easy to operate, even with gloves on.
Rubberized Zoom/Focus Knob:
The new zoom/focus knob is now rubberized to make gripping easier and more comfortable. Nauticam is the only housing manufacturer offering over-molded, rubberized parts. Like previous Nauticam handles, the built-in NA-A7 handles get the rubberized treatment that so many shooters have come to enjoy.
Nikonos Lens Support:

Old meets new as the Nikonos lenses finally get matched up with a digital camera that is worthy of these legendary water contact lenses.

Nauticam Housing for Sony A7 Review