I'm in the same boat as Tom. I'm a diver, and I have a 22, but I have not dived from the 22. I think it would be a somewhat mediocre dive-specific boat for a few reasons. One is that either the swim platform or a side ladder are not what I would want to clamber up on in full regalia. Two is that the cockpit is not very large, especially once you have all the "usual" stuff there already (cooler, fenders, lines, maybe a chair, etc.). So stowing tanks would be cramped.
That said, of course I think one could dive from it. Perhaps by donning/doffing gear in the water, or other means one might use from a smaller boat. But I just don't think it would be close to ideal.
As a diver, I look at the Tomcat 255 and drool! But for most of my other parameters the 22 fits just right, so if I do dive from my boat, it will be from the 22 and I'll accept the limitations. More likely I will snorkel from my boat and dive from (other) dive boats.
I think there are quite a few divers here, so you'll probably get some better (from real experience on a 22 diving) info.