I have the Tokina. I shoot Nikon so I don't have the Canon 10-22 or experience with it. I would think that it is a rectilinear lens What "rectilinear" means is that the lens presents a flat field onto the sensor. If you shoot a photo of a brick wall, the lines on the wall should be parallel to each other.
Now for the rectilinear lens, here are the probable attributes. To get a sharp image with it, you are probably going to have to use an 8" dome. You can probably use a 6" dome in a pinch. Either way, those are big domes and more than a bit clumsy. The reason for the big dome is that the lens does not focus really close. I would guess about 8" or so.
Now I would suggest going for the Tokina 10-17 which is a fisheye lens. Fisheye means all the straight lines in the photo will look bent. If you want to straighten them out, you can use post processing. I never bother. This lens is extremely popular for underwater photographers. I think that Tokina accidentally came up with a lens with attributes that make it uniquely suited for underwater photography. The pluses are very close focus like just a hair over 1". This means you can use a really small 4" dome which makes for a compact set up. Also, you can have a small subject in the foreground and the reef in the background. It can make for very nice compositions. The lens has a really nice depth of field. It also makes for nice funky shots on the boat after the dive. The bent lines and the usual goofiness of divers after a set of dives makes for a relaxed and funky looking set of shots. Also with the great depth of field, focusing is not a big issue on the rocking boat. You do lose a bit of zoom range to the Canon, but it probably is not enough to worry over. My suggestion if for the Tokina. But your appropriate choice is really determined by your taste, style of shooting and comfort. So my choice might not be the best choice for you.
