Shooting half and half pictures with a fish-eye lens can be successful when you use a small aperture and focus on the underwater section. At about f20 the above-water section will be included in the depth of focus. Apparently. But in my pictures the background seldom looks sharp enough if I am focused on a foreground subject, like a girl in a bikini. The distortion created by a fish-eye is also an issue for some subjects - like the aforementioned young woman.
Years ago I experimented with custom plano-convex diopters made in obscure strengths, like +3.3 for my 6" dome and +2.4 for my 8" dome. The "plano" aspect was meant to flatten the curvature of field but I found the B&W diopters were simply better.
On my 18mm lens I used split diopters for half and half shots in +3 and +4 strengths. Recently I have been playing with these again. One drawback was that once the diopter was mounted on the lens, you were committed to horizontals or verticals for the entire dive. No longer.
The Nauticam zoom gear I have has a sleeve about 85mm diameter. 3" drainpipe here in Thailand can have an internal diameter a shade less. Heat the end with a blow-lamp to soften it and you can push it over the Nauticam sleeve. You can pull it off afterwards, but it's a firm fit.
The 77mm filter holder I have has an outside diameter of about 80mm, so a 5mm long ring of PVC pipe glued in the end and sanded a bit acts as a strong push fit on the filter holder. The castellation cuts at the end allow for slight expansion.
Mount the filter in the pipe, then screw it onto the front of the lens then mount the camera normally in the housing. Paint the PVC with a black felt-tip pen to avoid reflections.
Ideally, have the dioptre mounted so that it works in the horizontal position with the filter almost tight so that only a 90 degree turn is required to put it in the correct position for verticals. As for useful dioptre strengths for half and halfs, that's another story!
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