The Archon V10S is the latest version of this popular LED underwater flashlight. It has a Cree XM-L T6 lamp bead, high brightness and long lighting range up to 300-400 meters. T6 lamp beads are very bright, but also more durable and longer lasting than other types of LEDs. The flashlight has a waterproof depth rating of 150 meters, which makes it suitable for diving or fishing (among other things). The light is also shock proof. If you drop it when diving, it won't break as easily as other lights would.
The ARCHON V10S Titanium LED Underwater Flashlight is the product you didn't think you need, but once you have it, something you won't want to live without.