I carry two lights- a headlamp and a regular flashlight.
I recommend the Surefire G2 if you want a BRIGHT light. I carry one and a headlamp with several light levels.
The headlamp- http://www.amazon.com/Rayovac-Sportsman-LED-Headlight-Pack/dp/B001F0RCIC
is great in locks-handsfree- so you can handle lines. Wherever you look- light. It has a bed low intensity setting for map work on the bridge. Got mine at WalMart for $15
I’ve had it two years- no problems.
The Surefire ran me $30- but it’s great if you need to get someones attention-fast- or for security at the gangway at night.
You can also mount one on a long gun or pistol with adaptors available from the company:D