A dive light is an essential piece of kit and picking the right one for you can take a bit of time and consideration. The most obvious task of a torch is to produce light in order for you to be able to see what you are looking at, especially in low light conditions. Lets not forget the other important use though...to add back the colour that is filtered out as sunlight passes through the water. The choice is huge and quite overwhelming if you are not sure what you are after or what is best. There are some simple questions you can think about to help narrow down your options though.
What type of diving do you plan on using it for? This question will help the most to filter down your list of potential dive lights. If you only intend to dive during the day in good light conditions and reasonable visibility you might only need a small handheld torch that is equipped with a fixed narrow beam light for poking in, around and under rocks to find any life that may be hiding there. A small torch can also be stowed easily in a BCD pocket, are light and pretty cheap to run.
Very low light and visibility conditions are best dived using a more powerful tight beam light that cuts through the murk whereas a wide beam light would be like having your car main beam headlights on in the fog, creating back scatter that lowers your visibility even further. Night dives, especially in good visibility, need a wide beam that will spread the light around and allow you to see more without having to constantly move the torch around. The video and colour definition from sport cameras such as GoPro or Intova Sports HD can be greatly improved using a suitable very wide beam light that emits a very consistent beam without hot spots. The beam needs to be far wider than most torches can produce to prevent a bright spot appearing, a 90 degree beam should be sufficient for most wide angle video cameras.
A dive light for technical diving needs to be very reliable, durable, incredible bright with a tight beam and have a long burn time. Most technical divers will opt for an umbilical option to allow for optimal light positioning and extra burn time that a separate battery canister can provide.
What lighting technology? LED (Light Emitting Diode) is very much the norm these days as recent technological advancements have significantly improved the brightness, reliability and size of LED lights. LED dive torches have proven to be so popular thanks to their power efficiently and reliability. The massive efficiency benefits allow manufacturers to produce lights that can burn at full power for hours on end from a comparatively small power source whereas you might be lucky to get a couple of hours out of a light that is four or five times the size that uses an older filament bulb. LEDs also lack the fragile filament found in halogen or HID bulbs that heats up and emits light, allowing most manufacturers to be able to state that the LED bulb itself has a life span of up to 10,000 working hours.
Unless you have a specific reason to choose a halogen or HID dive lights it is a wise bet to go for LED as your light source.
What power source? The battery that powers your torch is another consideration. Off the shelf you will have the options of rechargeable or non-rechargeable versions. Rechargeable dive torches are great for divers who are frequently in the water using them, allowing them to make the most out of their lower running costs. That said, they are not always the most convenient as you'll need access to power and also time to leave them to charge. Rechargeable batteries are not the best for holding a charge whilst in storage either so if you don't plan on using the torch regularly you might want to consider a non-rechargeable version. Non-rechargeable dive torches will typically be shipped with standard alkaline batteries which are relatively inexpensive and replacements are available pretty much everywhere in the world. If you want a bit more performance and burn time you might want to consider upgrading the batteries to the lithium equivalents as long as your torch supports them. There are obviously other things to consider before you buy your dive light such as body material, grip type and switch type but the simple question above should start you on the way to choosing a dive torch that suits your needs and the type of diving you do.