The design of a contemporary automatic dive watch with a highlighted 15 or 20 minute marking on its bezel is the result of copying a Rolex bezel design of the 1950s. Back then divers typically planned a dive to a certain maximum depth based on now obsolete US Navy dive tables, and dived according to the planned dive profile. If the dive profile allowed a bottom time of 35 minutes the diver, upon entering the water, would set the marker on the bezel, 35 minutes ahead of the minute hand. The diver calculated this with the 60 - bottom time formulae which looks like 60 - 35 = 25, for 35 minutes bottom time the diver would align the 25 minute bezel-mark with the minute hand. Once the minute hand reached the main marker on the bezel the diver would begin his ascent to the surface. The 15 or 20 minute scale helped with timing the ascent and whatever safety stop the diver deemed necessary. For contemporary diving methods the 15 or 20 minute count-down bezel is now quite antiquated.
Most manufacturers recommend to divers that they have the pressure on their automatic dive watch tested by an authorized service and repair facility annually or every two to three years and have the seals replaced. Besides that, simple maintenance by the owner is also important. Most manufacturers recommend rinsing the watch in fresh water after use in seawater, but leaving a diver's watch in fresh water overnight is a good method to protect the watch from corrosion and to keep the crown, buttons and pressure sensors on digital ones working.
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