The Nikonos RS 13mm is a fisheye, so not really fair to compare to the WACP which is rectilinear.
I guess one thing to keep in mind is that the focal length quoted on the lens barrel is the "true" focal length, in air. This is a physical property of the lens construction. The actual focal length in use (underwater) must be multiplied by 1.33. This also changes the FOV of the lens.
I had long assumed that Nikon would have originally quoted the focal length (and therefore FOV) on the Nikonos RS and 15mm as the focal length in their native medium (water), since the lenses where designated as R-UW, and don't focus in air. I guess that is not the case.
I am assuming this does not happen with the WACP, and in fact, just the opposite. Because of the "Correction", the field of view is increased, at the same time "repairing" the damage done to the corners of the image due to refraction (CA) and curvature of field (DOF).
That's got to be some fancy physics. As you said... "You CAN have your cake, and eat it too".
That doesn't often happen in the real world:)
Thanks Ryan.. I just spotted your response while I was typing:)