Over the past 70 years, inventors have come up with scores of innovations in an effort to improve diving watches. In the early 1970s, for example, a Japanese inventor named Yoshiaki Fujimori came up with a new type of ultra-stiff seal ring that was less likely to deform under pressure than a rubber seal [source: Fujimori]. In 1988, another team of inventors from Japan unveiled an electronic diving watch that included a water pressure sensor [source: Utemoto]. More recently, in 2010, French inventor Jean-Francois Ruchonnet received a patent for a depth gauge that would fit inside a diving watch [source: Ruchonnet]. Swiss inventors have even developed a specialized diving watch that, when combined with a sensor attached to the chest, continuously monitors a diver's heart rate [source: Angelini].
To withstand great pressure and protect the watch mechanism, the cases of dive watches are made from extremely hard, stiff materials [source: Omega]. One popular choice is stainless steel, an alloy whose coating makes it resistant to being deformed by water and succumbing to pressure as well as corrosion. (It's so tough that it's also used by some automakers to make cars' structural parts more crash resistant [source: European Stainless Steel Development Association].) Titanium and 18-karat gold are other common choices.
If you're actually going to wear a diving watch for diving, rather than just because it looks cool, be sure to get one that conforms to ISO 6425, the internationally-recognized standard that certifies a watch is capable of resisting pressure, moisture and works underwater for extended periods of time. To earn that distinction, a watch has to function while immersed in water for 50 hours. Additionally, after being heated to up to 113 degrees Fahrenheit (45 degrees Celsius) for 20 minutes, testers drip water onto the exterior of the watch face. If condensation develops on the inside, the watch won't pass. If the watch is rated for 656 feet (200 meters) of depth, it has to be able to withstand 125 percent of the pressure found that deep, and keep working for a two-hour period [source: Avionica].
ISO 6425 also requires a watch to have certain important functional features, such as a rotating bezel, a ring around the watch face that allows a diver to mark five minute increments of time. The watch also must have a luminous tip on the second hand, so that it can be seen even in the dark ocean depths, and continue to function when exposed to salt water and to the presence of a magnetic field. It's also required to withstand a blow from a 6.6 pound (3 kilogram) hammer at an impact velocity of 14.4 feet per second (4.4 meters per second). The watch band undergoes similarly rigorous tests, to ensure that it won't snap under stress when it's in deep water [source: Avionica].
Depending on how much money you have to spend, there are other features you can get as well. Some dive watches include multiple gauges that measure elapsed time on the bottom, and have alarms that flash or vibrate so you can sense them even when water muffles sound. Others are made from high-tech ceramic composites -- the cutting edge of pressure resistance -- instead of steel or titanium. And some even have special anti-glare crystals on their faces [source: Naas]. It's your choice which features fit your diving needs!