Shane,

I did that same shot about a 2 years ago. Girl in pool swimming up to the surface; jib under water booming up to catch her reaction as she breaks the surface. Budget didn't allow plus we we're shooting film. So we attached a 24" speedrail to the head, eleminating the tilt and using only the pan. Mounting the water camera on a cheese plate we we're able to adjust the tilt for the end shot form the ass't in the water and yet still having the freedom to pan. All the motors we're out of the water as only the camera and speedrail were under. There's something to think about: the weight difference from under water to surface is extremely different. To get the camera under is less weight, however to get it out on to the surface requires to guys in the back to lift the jib out.
This was the quick fix and it worked, but this was also before ALS's design. I believe he also cut holes in the front jib tube to allow water passage. We got the shot with a little timing......good luck.