Probably the best current option is the new Inon UWL-H100 M67 and of course would be even better with the Dome II kit. Very expensive, large and heavy and should work at 28mm without vignetting or excessive edge distortion, my bet. The older UWL100-67 with dome would be a good choice as well, would require zooming to 35mm, also large, heavy and expensive.
INON Wide Conversion Lens UWL-H100 28M67 [Overview]
This should work, I think, I know it will with the Canon or FIX or Fisheye/Nautica S series housings:
INON NEWS: Underwater Micro Fisheye Lens [UFL-M150 ZM80] is released
Not a wide angle but a macro fisheye, I want one really bad, it may be the answer to boring macro.
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