Hi, Mark -
My fiance and I have used 8-15 mm lenses since September. Dive sites have included the Solomons, Curacao, and our local CA waters, and my guess would be that we've done at least 60 w/a dives in that time. We have two housed Canon 7Ds and a housed 5D MkII between us, and since we almost always dive as a team, to us, this lens is very versatile!
On a 7D, it is our opinion (as demonstrated by Ryan at Reef Photo et al) that this lens is sharper than the Tokina, and has almost the same zoom range (you will be limited a bit in comparison, but not much). It has given us beautiful results so far, and I very rarely zoomed my Tokina much anyways, so it's been a nonissue. Since I got my 8-15, I have only used my Tokina once, and that was because I wanted the zoom flexibility during a particular dive and didn't have a zoom gear compatible with the housing I was using for my 7D.
Just in case you ever plan on diving a full frame, I'll add this, too... While the circular aspect is a bit - well - gimmicky at times, it can work very well to create beautiful, incredibly unique images with certain subjects. I believe Alex Mustard has said here before that he sees this lens on a full-frame camera as either an 8 mm or a 15 mm lens, and I agree. Since my UW option on the 5D right now is a 15 mm FE, that's no biggie. We're still figuring it out at 8 mm - it does seem that there is a bit of chromatic aberration (with OUR gear) at the 8 mm end, but it isn't noticeable unless you really magnify your image and is easily fixed with a slider in Lightroom. We haven't done any detailed comparisons yet between the 8-15 at 15 and the Canon 15 mm FE - I haven't personally noticed any difference, and I haven't put the 15 mm FE back on since I bought my 8-15!
If you have the money, I'd go with the 8-15, but you could always start with the Tokina and upgrade later. The Tokina is an awesome lens!
Good luck with your decision -
Allison
Edit - you posted just as I wrote this - congrats on your new "baby" - enjoy it!