I've been hoping to try an UW pano myself. Topside panos and over/unders seem to sell well for me. Maybe i'll try an over/under pano next time.... 
Rotating around the nodal point of the lens will be very important methinks. Especially with how close we must get to things UW, get some serious parallax. A pano head would be a good idea but they're usually expensive, not designed for UW and generally too small (with nodal point gear) to accommodate a housing. I wouldn't dare take my Really Right Stuff pano gear UW ($$).
Some sort of panning head and a custom nodal point rail are the best bet, and what i'm working on now. I have a cheap velbon plastic video tripod with a pan/tilt head i've used UW a few times in the past couple years. The head still works just fine, though screws are rusty.
Karel's done an excellent job and luckily the G9's lens nodal point will be close enough to the base of the housing that it can work ok mounting the tripod there. With a DSLR it will be more difficult.
If I remember correctly from Claude's awesome submarine, he shot it by swimming down the length of the sub and taking flat-on shots. This is not the traditional type of panorama taken from 1 spot. Unless you're subject is very flat (which he managed to achieve visually) with a very uniform background/foreground like sand that can be blended together without matching up, one will run into major stitching issues due to changing perspective.
Cheers,
Chris