This strob and flashlight combination unit from Subgear is a great accessory for night diving.
The Strobe section has a range of Approximately 2KM and is a superb emergency beacon/signal strobe, on the surface.
Or use the strobe to identify divers or shot lines etc underwater.
This improved version has an LED lamp on the opposite end providing a great back up light, or fix the strobe to your BCD shoulder strap and the lamp shines down to illuminate instruments and the strobe is clearly visible!