I got to try a Nite Rider 10W HID light yesterday. The battery container is flat and rectangular, and was very easy to attack to the waist belt of my BC and sat comfortably there. The light head was TINY, sat in a soft wrist bracelet so easily that I was almost unaware it was there. The light was very good, although the beam is a little diffuse for punching through Puget Sound murk, compared with my friend's Terkel light.
According to my LDS, these lights are used by the police divers in our area because of their excellent durability.
This is high on my list of possibilities for my imminent light purchase.