(Free)diving w. 5D2. Hugy or Aquatica. Best choice...?

I was pretty set on a fixed 20mm, but then I started reading about rect. lenses and lack of corner sharpness, so perhaps the Tokina 10-17 w....

You can't use the Tokina 10-17 fisheye on the full frame cameras. It's for EF-S mounts, not EF.

Yup, you are basically right but I have seen here on WP that people have begun mounting them on 1.4 extenders and then one can shoot on a full frame body. Seems like 10mm could vignette, but everything else seems good from the samples I have seen here.

D.

You can't use the Tokina 10-17 fisheye on the full frame cameras. It's for EF-S mounts, not EF.