Remember the scene in Jaws when Hooper (Richard Dreyfuss) and the Chief (Roy Scheider) went out at night looking for Bruce, the great white? That was the first experience for many people with underwater lighting on a boat. Divers have known about underwater lighting for years, and there isn't a squid fisherman alive who hasn't lit his boat like a Christmas tree to draw those tentacled critters to it.
A relative newcomer to recreational boating, underwater lighting has enjoyed increased popularity over the past two years, mostly on larger luxury craft. Like many things in industry, the benefits of economies of scale eventually trickle downward: Underwater lights have started decreasing in price and size, making them more accessible to more boaters.
HID and LED
Consumers needn't worry about much underwater-lighting terminology. Essentially, you need to know the two types of lights and - to a lesser extent - how to compare them before purchase. Seeing the lights in action on a boat constitutes one of the best means of comparison. Failing that, you'll need to know lux and lumens (terms that define brightness) and what they actually mean.
HID (High-Intensity Discharge) Lights
These commonly encompass at least four distinct types of lamps, but the underwater lamps you'll deal with are metal halide types. These work like every other commonly known light bulb (save LEDs - more on those later): Electric current arcs through a mixture of gases, producing light.
In a metal halide lamp, the gas mixture - composed of argon, mercury and a variety of metal halide - heats up from the electrical arc, vaporizing the mercury and metal halides and, poof: bright, white light.
We use HID lamps for general lighting or when the application requires specific UV or blue-spectrum light. Since blue and green light travel farther underwater than red and yellow, most anglers prefer the former. (There seems to be significant agreement that green light works well as a fish attractant. The aforementioned squidders have used green lights seemingly forever, as have swordfishermen and commercial longliners.) You also commonly find HIDs at athletic venues.
Some HID manufacturers include DeepSea Power & Light, IMTRA, Sea Vision, Sea Waterlight and Underwater Lights Ltd.