Hello Lundysd,
Dont buy a 10 Watt HID, you'll end up deciding a few months later that you need a brighter light. I made this mistake myself, I bought a Light Cannon 100, it showed me HID was the way to go but three months later I upgraded to a 50 Watt HID and have used and loved it for two dive seasons now.
Go for a 21 Watt or above and look for a maker using Brightstar bulbs, theyre more robust that Welch Allyn bulbs made for the medical industry.
Take a look at the Metalsub HID125 (24 Watt HID) and a 5 Ah or 9 Ah battery pack.
Check out
www.metalsub.nl for more info, and
www.metalsub-usa.com for sales.