Ghostpipe,
Also - I know you have eliminated the Nikon 16mm from your list but I used that lens on hundreds of manatee shots and it is a great, great lens - not quite wide enough on my 1.5x for all the manatee photographs I wanted to get but still wide enough to accommodate most shots of those large creatures.
I now use the Tokina 10-17 almost exclusively on the manatee and I never get any vignetting on the Subal 8" dome, even at 10mm. It is fantastic to have the versatility to zoom but I do miss the Nikon 16mm as it was clearly a better lens and I do have a little more trouble with the Tokina 10-17 in the low light areas of the springs. I use a Nikon d200, 1.5x.
I've never used the 10.5 as I didn't want to get locked into that wide with the manatee (which is 85% of my WA stuff).
Just my experience, Carol