I would look for 2 things:
- a narrow beam (6 - 10 degree beam angle). Narrow beams are good for signalling. Wide beams are all but useless (EDIT: for signalling!). Using your light for signalling at night is an important function.
- adjustable brightness. A lot of the time you won't want it to be super bright, as that can scare or annoy the critters.
My favorite for all my diving is the Xtar D26. They are about $63 on eBay (plus the cost of a battery or two and a charger). They have a nice, narrow beam. They have adjustable brightness. They are super bright, when in High or Turbo modes. A single charge should easily last 2 dives on High.
Update: They are even less expensive now! $55 for just the light.
Xtar D26 L2 U3 1100 Lumens Diving LED Flashlight -- Simple Kit | eBay
Or $75 for a kit with battery and charger.
Xtar D26 L2 U3 1100 Lumens Diving LED Flashlight Full Set w/Charger and Battery | eBay
That battery and charger are not great, so I would get the light by itself and buy some Soshine 5500mAh 26650 batteries and an Xtar charger.
2 batteries for $15
Soshine 2pcs 26650 3.7V 5500mAh Rechargeable Li-ion Battery PCB Protected U5R9 | eBay
Charger for $23
XTAR VC4 USB Charger Li-ion/Ni-MH 18650 26650 AA AAA + USB Wall & Car plug
Alternative: Charger/tester for $90, and worth the investment, in my book
XTAR DRAGON VP4 PLUS PREMIUM Li-ion/Ni-MH LCD Charger
Is this all enough to fit into a BCD pocket???