Whether you're a scuba diver or a free diver who depends on lung capacity, "no diver should enter the water without a reliable timekeeping device," scuba-diving experts and physicians Michael B. Strauss and Igor V. Aksenov write in their 2004 manual "Diving Science: Essential Physiology and Medicine for Divers." For breath-hold diving, monitoring both your time underwater and the amount of time that you're resting on the surface between dives is critical because it enables you to keep within the precise parameters your body can handle. Additionally, monitoring your time at the bottom enables you to calculate precisely when to start your gradual ascent, which reduces the danger that you'll black out before you get to the surface [source: Strauss and Aksenov].
With scuba, where you're likely to descend even deeper for longer stretches and are dependent upon a limited air supply, the need to keep careful track of the time is even more crucial. That's why dive watches are designed not only to withstand the extraordinary stresses of the underwater environment, but also to dependably provide critical, potentially life-saving information to divers and to be as easy as possible to use underwater.
Those considerations went into the first watch specifically created for serious diving back in the 1930s, the Omega Marine. To protect the watch's time-keeping mechanism from temperature variations and pressure, designers enclosed it in a then-innovative stainless steel case. To ensure the watch's durability under extreme stress, researchers subjected it to rigors more intense than any diver likely would face -- immersing it, for example, in water heated to 185 degrees Fahrenheit (85 degrees Celsius) for several minutes, and then abruptly plunging it into near-freezing water. In another experiment, they lowered the watch to the bottom of Switzerland's Lake Geneva, a depth of 240 feet (73 meters), and left it there for 30 minutes. Ultimately, in 1937, the Swiss Laboratory for Horology certified the watch as able to withstand pressure 13.5 times what it experienced on the surface. Last but not least, they persuaded underwater explorer Charles William Beebe, who at the time held the world record for ocean depth, to take the watch for a test dive [source: Omegawatches.com].