I've had heard comments that the Sigma 14mm has flare problems that severely limit its use underwater. I do not have this lens but I have owned the Tamron 14mm for quite a while. They seem similar physically, as do all the 14mm lenses.
All options to the 14mm will have significant distortion I'm afraid. They will also have much less severe front curvature and better flare control. I don't know about the Canon 15mm fisheye. I believe James has used the Nikon 16mm fisheye. If so, how severe is the distortion?
As an aside, everyone seems to support the Sigma 14mm but none of the others. Is there a technical reason for this? A conspiracy? I suspect its because that's what they have on the shelf.
No one seems to be pursuing wide angle with color filters. This is the area that interests me the most!