Underwater housings are a tricky beast and are largely going to depend on acceptable cost and features that you need. Personally, my recommendation is to take a look at B&H's selection of housings available for your camera. There aren't a whole lot of options available in the segment (only probably about 5 or 6 enclosures even made for any particular model and several of those don't provide access to all controls.).

If you intend to use a strobe or light, Ikelite makes some reasonably "cheap" ($2200 - $2400 with ports) options, though not quite the cheapest, but they integrate well with their underwater strobes and lights.

The cheapest reliable option that I've seen is made by Equinox. It runs around $1500 and looks basically like a cylinder with knobs on it and a clear front and back to see the camera and for the lens to see out. It's not very versatile since it has to be made for one particular body and lens combination, but it's about 2/3 to 1/2 the price of a modular system.

Aquatica also makes nice enclosures (I believe their's was the one I was favoring since it could support manual focus, which is key for doing video underwater.) They are however, more expensive than Ikelite.

I don't have any personal experience with any of the options, but as a fellow diver and photographer (with a 5D Mark III), I've just recently been looking in to the same thing.