I have to disagree with the difficulty issue about the 10.5 that mantion here from time to time, its sure more challenged lens from the less wide ones but its not so bad <_<
From the beginning I almost didn't took pictures of my fins as I supposed to B), it's happens only when I'm doing some strange positions in the water trying to take difficult frames.
Its sure more challenging to fill and light as mentioned but a little practice and it can be done spectacular results that the 12-24 can't do.
When I said challenging I was thinking of the lighting and finding a suitable (big, close) subject, rather than keeping yourself out of the frame 
Of course this lens is good only for really wide angle shots and you have to check if it’s fit your needs.
Definitely. And the 10.5 is good, but you can still get the big scenic wide angle with a 12-24, plus you can shoot the medim - large fish. Plus you don't have to be so careful about where other divers are in your shots.
The fact you could add the 10.5 to your aresenal with the same port as a 12-24 I think adds more weight to the plus side of a 12-24. I started with a medium wide angle back in the film days, and loved it. While the fisheye adds to my kit - it isn't the only wide angle lens I'd like to have.
Ah Tim, you haven't realised what you are missing in JB
... green, cold, and chunky now though 