On Tuesday I tested everything we had on hand. This was a quick test in a utility sink, so no images to post, but it lets me eliminate the garbage.
The clear winner was the FIX UWL-04
M52/67. The lens is threaded M52, but threads on the housing with the supplied M67 adapter.
FIX UWL-04 Fisheye Wet Mount Conversion Lens - 67mm/52mm [fix.uwl.04.m52/677] - $399.00 : Reef Photo & Video!, The Underwater Photo Pros
This lens vignetted slightly at 28mm, but one bump of the zoom lever eliminates it, and you are left with what I'd guess to be 120-130 degrees diagonal field of view.
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Inon UWL-100 Type 2 and Ikelite W-20 / Epoque DCL-20 both have no vignetting at the 35mm zoom setting in water. Based on experience with similar setups in the past I'd expect some corner softness, though.
Inon UWL-100 28 AD had some vignetting at 28mm, but no vignetting at 35mm. I'm interested in doing some comparisons at some point between this lens and the UWL-100 when I have a grid chart setup in the pool.
How is the FOV between the Inon UWL-100 28 AD and the FIX lens you called as the clear winner? And how do they connect, to which housings? These seem like the top contenders to me, not the Inon UWL-100 Type 2, based on your description. (Especially once you add the Inon Dome Lens Unit for UWL-100, at which point I'd think it should technically have a wider FOV than 120-130 degrees)