Trto keep it short about torches....they key thing is not the ultimate power but the spread of light. Over here, in anything but absolute perfect viz, which I haven't seen for at least 8 years, you need a focused beam. A wide spread will just "white out" on the murk no matter how much power you have.
Of course, as long as it's focused, the more power the better. The good thing about the D6 mentioned earlier is it has a large reflector meaning narrower beam. Combined with the XM-L led (small die size compared to previous high power leds like the P7), it will work well. The downside of it is the battery power. As it only uses 1 18650 cell it will barely last an hour at full power. Obviously reducing brightness will increase run time.
Anything with an XM-L in it will be pretty decent but do not be tempted by the triple xm-l ones on ebay. By cramming in the 3 leds the reflector have to be tiny, which means they will be a flood style light, which is not what you need.
You can get a triple XM-L to work but you need to spend yer cash on decent reflectors and build a torch around them, and by doing that you can add a massive battery pack on an umbilical for loads of run time, but that takes plenty of time and research!
Personally I prefer hand held to wrist mounted, I find it easier to point where you want rather than moving your whole arm. If you have it on a short leash you can drop it and do your tidying up work in the silhouette of the light projected below.
Night diving general, go places you've dived in daylight, keep your head up to keep your bearings on shore light, and yes, take a compass. I've only used mine once, when an unexpected fog came in all of a sudden and I could see nothing. Without it I'd have been left trying to work out my way back based on knowing which way the ripples in the sand usually run....hardly a foolproof navigation technique!