Underwater, a steady beam of light is useful for illuminating underwater critters and signaling your buddy. But what is the purpose of an emergency strobe? What can it do that can’t be accomplished by waving your dive light around until somebody notices?

That depends on how long you sit there waving your light around. On their highest power setting, most dive lights only last a couple of hours. This is more than sufficient for a typical recreational dive. Even if you need to signal for assistance at the surface, once you’ve been spotted, you should be out of the water with burn time to spare. But what happens if no one sees you? Or if you surface too far from the boat or shore to be seen? Then you could wind up being adrift longer than your battery will last. Much longer.

Related Reading: A Beginner's Guide to Using Underwater Strobes

That’s where a strobe or emergency flash mode comes in. They don’t make your light more visible, but they do increase its run time. Alternating between on and off allows lights to significantly save battery power and run for an extended period of time, even on high power. This can effectively double the length of time your light can run, in some extreme cases it can extend operating time from hours to days, allowing you to signal rescuers long after you’ve drifted off.

A light with an emergency flash or strobe is an important tool to bring on any open water dive, even those that take place during the day. If you get drift away from the boat or shore and are not found by the time the sun sets, a light will make you stand out like a lighthouse against the darkening sky and sea.