Wide angle rectilinear lenses like the 7-14mm produce serious distortion, particularly in the corners where the fabric of the universe gets stretched very significantly. Fisheye lenses don't suffer from this type of distortion, but vertical and horizontal lines are curved as one moves away from the centre. Before digital, these were really the only choices. But now there is software that corrects both wide-angle rectilinear lenses and fisheye lenses in ways that eliminates bent verticals and stretching as one moves away from the centre. These corrections are not severe and so image quality is excellent post correction. For fisheye-lenses there is:
Fisheye-Hemi ( http://www.imagetren...odpage_hemi.asp )
and for both fisheye and rectilinear lenses there is:
DxO Optics Pro ( http://www.dxo.com/i...me-anamorphosis )
But it's only occasionally i see underwater images that i object to because of such distortions (so correction is seldom needed, but available.)
Because of this, it's more a field-of-view question about which lens to use and the advantages of having a zoom underwater when you can't change lenses. The 8mm fisheye is much wider than the wide end of the 7-14mm, and you can also use the 12mm f/2.0 behind the 4.33" dome for the 8mm (or the 3.5" dome w/extension). It's the option i've chosen, because it's smaller for travel and because above water i prefer to use faster primes.