Thanks mate, great inspiration. I have two Ys-90's and after seeing your bounce flash stuff was thinking about using a dive slate, much like I would use a white card on land. The Light table stuff is insane!
Remember - light doesn't travel far in the water. You're still gonna have to get really close.
I've used the sand a number of times to light the underside of an Electric Ray or my buddy as she was cruising close to the sand.
On the shot below, I used a slate here to get that spooky reflection in her mask, while lighting up the underside of her as she was doing a reef-check survey.
Try stuff....
-Ken