As a caver, I've seen and used all manner of small efficient lights. Here's my opinion on white LED's:
I own a Petzl Tikka (3 LED's, 3 AAA's), a Princeton Tec Matrix (3 voltage regulated - dimmer initially but constant brightness throughout battery life, 2 AA). I have used single LED flashlights, and array LED lamps.
The Tikka is an excellent headlamp. It's very light and bright, but the best part is that there's a very even distribution of light... sort of like the light from a candle or carbide lamp, just bluish. The Tikka is excellent for camping, while the Matrix (similar headlamp, but more rugged) is great for caving in low-volume passageways.
I'm unimpressed with the LED flashlights. They're wonderful for reliability and have very even light output, but just don't have the intensity from an incandescent flashlight. You simply can't beat a decent 4 watt headlamp for lighting up a decent sized room, or shining a beam 60 feet to the ceiling. You just can't do that with an LED lamp.
This being said, I can't imagine white LED's making a decent scuba light. I could see an LED array making a nice even light that illuminates about 3 feet in front of you... 20-LED arrays begin to rival perhaps 5 or 6 watt halogens, but (especially given the cost) cannot compare to a 10 watt HID even a regular 12 watt halogen.
Then again, I've never used LED arrays underwater, I may be completely wrong