Recommend you look at your new underwater camera as a complete system. Depending on the type of images you want to make, the lenses, ports and strobes can get pretty expensive, much more money than a body. Folks who grab their camera and kit lens and put them in a plastic housing tend to be disappointed with the results.
Shooting underwater is very different than on land. To capture the images you probably have in your mind’s eye you’re going to have to get very close to limit the amount of water between the subject and the camera. This means a wide angle lens like the Canon EF 8-15mm Fisheye or a Tokina 10-17mm on a crop camera is the way to go. These lenses require relatively pricey domes to get great corners. For macro shooting the 100mm macro is the way to go in clear tropical water. The new one focuses a little quicker than the non L older version.
We know that photography is all about the light and that is even more important underwater. One good strobe is enough to get you started but you’re going to want two eventually, especially if you love shooting the wide angle stuff.
The 6D looks like a nice body for underwater use with the cautionary exception of the low sync speed. 1/180s is a touch slow to capture those great sunray images everyone loves. It’s possible but harder than using 1/250s on a 7D for example. The 5D II & III is at 1/200s so a touch better. Just one of many things to consider. If you travel long distances to dive and shoot it’s pretty common to carry two bodies just in case. Saltwater and electronics don’t exactly get along.
If I was you I’d seriously consider a 7D or 5D Mk II and pickup a used high quality aluminum housing for it. (Aquatica, Nauticam, Sea & Sea, etc) Spend the money now on lenses, ports and strobes that you can grow with. When the 6D comes out, and the housings finally start to ship, or the MK III comes down a little, you will be in a great position to make an upgrade choice in a year or two. If you’re living right the new 7D MkII will come out with the controls in the same place and your housing will work with the new camera. (OK, wishful thinking)
Most importantly, spend the time to go to one of the top level shops that specializes in underwater equipment. Have them put a whole system together and price it for you. Lay your hands on the different housings and spend some time with them. Feel how your hands lay on the controls. When you’re at a 100 ft and the shark is circling in to see what you’re up to it’s no time to have to look to find the damn shutter speed dial.