Essentially, a dive watch has certain characteristics that make it safe for diving, with the most important factor being water resistance. The International Standardization Organization establishes the standards for dive watches and set the ISO 6425 standard in 1996 to govern this realm. ISO 6425 clearly states that a dive watch must be able to withstand depths of at least 100 meters while still keeping time accurately. This is a minimum, though, and many watches are water resistant to 200 or even 300 meters thanks to screw-in crowns and screw-down case backs.
Some diving watches may have a helium escape valve so they can go even deeper and then vent upon ascent. A helium escape valve allow built-up gasses to release from the watch in order to prevent the sapphire glass from removing during decompression. It is important to note that helium escape valves are typically only necessary in watches used by commercial divers who may need to spend days living under pressurized conditions.
Requirements for mechanical dive watches that are slightly different from quartz or digital dive pieces. These include the fact that the watch has a diving time indicator (usually on a rotating bezel) that allows for reading of dive time. Additionally, the minute markings should be clearly readable to demonstrate at a glance that it is running while underwater and in darkness. For this reason, most dive watches feature a running second hand with a luminous tip. A good dive watch should also be anti-magnetic to 4,800 ampheres and still be accurate to plus/minus 30 seconds a day.