I need to clear up a little misinformation here about HID lights, add a little more data, and add another lighting technology.
First, HID lights vary in quality by bulb manufacturer as well as balast. You can run a 21W HID bulb at 18W, 21W, 25W etc. Depending on how stong your balast runs (as designed by the manufacturer), you will get warmer or cooler light. The spectral qualities of HID are not simply 'blue'. The light is really 'white'. The bulb manufactuer has the say in the spectral quality - they will add various doses of metal halide salts to produce the spectral quality of light, as well as flicker & flare stability, of their product. For bulbs manufactured by Welch/Allyn, the 21W HID bulb has most of its spectral output in the 400nm to 700nm range (the visible range) with a dip at 575nm and a spike at 625nm. The color fidelity produced by this lamp is quite good across the visible spectrum.
The important issue to watch out for with HID is its 'turn on time' or the time it takes to get to a stable light intensity, which is in the range of 15 to 20 seconds. This is important if you see some once-in-a-lifetime shot and your lights are off. On the plus side, HIDs burn physically cooler in temperature than halogen, which is directly related to the much lower battery juice requirements of HID. In my opinion, HID winds hands down.
A few dive light manufacturers to have a look at are Halcyon and Sartek Industries. The both have roughtly equivelent video lighting systems with a variety of battery packs to choose from, including the support of wet connections if you so choose. In my case I went with a pair of Halcyon Apollo 24W HID video lights with the 9ah battery pack (approx 100 to 120 mins burn time with both lights powered). The battery pack is cliped to my BC and a wet-connect attaches to the lights on my housing. This makes the housing a little less negative/bulky. The wet-connects also allow me to ditch the rig in an emergency.
The high quality (and expensive) bulbs used by both the Halcyon and Sartek systems are manufactured by Welch/Allyn. They have a web site with specs for their bulbs, including the 21W bulbs used in the above mentioned UW lighting systems. In particular, take a look at the 'Solarc Single Ended Lamp Performance Data' on the tech-specs page for the 21W bulb (doc titled 'LSL009E.pdf').
Last but not least, is the HMI lighting technology - very expensive, very bright at 50W minimum, very stable and repeatable color temperature with a very white light output. These are the types of lights used by film makers to light up their sets in the UW world. The big-boys go to Hydroflex to get their UW lighting needs satisfied. For some fun and educational reading, go to their site and take a look at their 'practical applications' pages.
Hope this all helps,
-Not affiliated with any of the mentioned companies
Links to Halcyon & Sartek Industries:
Link to Welch/Allyn:
Link to the 21W bulb specs:
UW Lighting for the Big Boys: