After reading Alex Mustard's article in UW Photographer and checking out several of his posts here on WP I wanted to give this technique a go. During a trip to Grenada I experimented with a variation on Alex's equipment. I didn't want to purchase anything additional so I limited myself to equipment I already owned so what I ended up using was a Sigma 15mm FE, Kenko 300 Pro 2x tele, 8" aquatica dome with the extension for my 17-35 mm Nikon (#18457 - 67mm.)
What I quickly discovered was that because of the light loss at 2x and generally darker conditions on the dives where I tried it, I was having difficulty getting enough light to get a pleasing available light background. The depth of field was not as great as the examples posted by Alex. This is not to say everything was bad. Overall sharpness as well as corner sharpness was better than I expected and I was able to focus on and light subjects very close to the dome.
In conclusion I think this set-up would be well suited to the type of cold-water subjects I normally photograph locally - lobsters, crabs, anemones, wolfish. I will definitely make this a priority on my next trip to Eastport. For tropical waters I wasn't able to get the DOF or nice backgrounds that I saw in Alex's shots but I intend to keep this as a change-of-pace technique when I get tired of the same ol' look.
Here are few of the shots.
