I shoot super-macro with a 5D Mk III in an Aquatica housing, with two large Ikelite strobes. You position the strobes immediately on either side of the housing (to start), then work them around, depending on the effect you desire (shadowing, back-lighting, etc.). Have some examples at www.DavidPriceImages.com.
To shoot the larger-than-life stuff, you either need extension tubes, or an external diopter.
With extension tubes, you have no autofocus (it is literally a gap between your lens and the camer body). With a diopter, they are generally "wet", meaning they go on the outside of your port, and are often in a hinged holder, that will swing the lens out of the way when not in use (Xit 404, Subsee, and many other make them). The diopters (like SubSee, SMC, etc.) fit into this holder. The diopters range from +5 all the way up to +17.
You can also use a diopter directly on your lens, inside the port (if it will fit). The main drawback is you have to shoot everything with the diopter, as it's mounted directly to the lens.
Autofocus becomes an issue with a diopter, as your depth-of-field gets very small. The way most people do it is to use "focus lock" and get the image close, then move the camera very slightly back-and-forth until the image is tack sharp. Be warned, however, that if you have a dioper adjustment wheel on your camera's eyepiece, that your image may not "actually" be sharp. Best to set everything up and test in a pool, first, to make sure everything that appears sharp in your viewfinder, actually is sharp once you get the image off the memory card.
Hope the above helps.
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