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A diving watch is a watch designed for underwater diving that features, as a minimum, a water resistance greater than 10 ATM, the equivalent of 100 meters (330 feet). The typical diver's watch will have a water resistance of around 200 - 300 meters (660 - 990 feet), though modern technology allows the creation of diving watches that can go much deeper. A true diver's watch is in accordance with ISO 6425, which defines test standards and features for watches suitable for diving with underwater breathing apparatus in depths of 100 m or more. Watches conforming to ISO 6425 are marked with the word DIVER'S to distinguish diving watches from look a like watches that are not suitable for actual scuba diving.

The ISO 6425 standard is equivalent German Industrial Norm DIN 8306. It provides minimum requirements for mechanical watches (quartz and digital watches have slightly differing readability requirements) like a unidirectional bezel with elapse minute markings, minute markings on the watch face, visibility at 25 cm in total darkness, indication that the watch is running in total darkness, shock and magnetic resistance, thermal shock resistance, strap/band solidity, End Of Life (EOL) indicator and a minimum of 100 meter depth rating.

The testing of the water resistance is fundamentally different from non-dive watches. Water resistance testing of a diver's watch consists of:

• Immersion of the watch in 30 cm of water for 50 hours.
• Immersion of the watch in water under 125% of the rated pressure with a force of 5 N perpendicular to the crown and buttons (if any) for 10 minutes.
• Immersion of the watch in 30 cm of water at the following temperatures for five minutes each, 40°C, 5°C and 40°C again, with the transition between temperature not to exceed 1 minute.
• Immersion of the watch in a suitable pressure vessel and subjecting it to 125% of the rated pressure for 2 hours, no evidence of water intrusion is allowed.

Except the thermal shock resistance test all further ISO 6425 testing should be conducted at 18 to 25°C temperature. The required 125% test pressure provides a safety margin against events like sudden pressure increase surges.

Watch case

The watch cases of diving watches must be adequately water(pressure) resistant and be able to endure the galvanic corrosiveness of seawater, so the cases are generally made out of materials like stainless steel, titanium, ceramics and synthetic resins or plastics. The case must also provide an adequate degree of protection against external magnetic influences and shocks.

Elapsed time controller

Analog diving watches will often feature a rotating bezel, that allows for an easier reading of elapsed time from a specific point and compute the length of a dive. On diving watches the bezel is "unidirectional", it contains a ratchet so it can only be turned anti-clockwise to increase the apparent elapsed time. If the bezel could be turned the other way this could suggest to a diver that the elapsed time was shorter than the truth, which could be highly dangerous. Digital dive watches usually perform this function by use of a standard stop watch function.

Crystal

Diving watches have relatively thick watch crystals. Sometimes domed crystals are used to enhance the pressure resistance of the watch. The typical materials used for crystals are acrylic glass, hardened glass and (synthetic) sapphire which all have their pros and cons. Acrylic glass is very break resistant but can easily be scratched. It has however the advantage small scratches can be buffed out with polishing compounds. Hardened glass is more scratch resistant than acrylic glass and less brittle than sapphire. Sapphire is very scratch resistant but less shatter proof than the other crystal options. Anti-reflective coatings are generally applied on sapphire crystals to enhance the legibility of the watch. Some manufacturers use sapphire/hardened glass laminate crystals, where the scratch resistance sapphire is combined with the better shatter resistance of hardened glass.

Crown

Analog diving watches must have a water resistant crown. Often the crown has to be unscrewed to set or adjust the time and date and afterwards retightened to restore the water resistance of the watch and minimize the chance of unintentional operation under water. There are however models that have crowns that are operated like the crowns of non diver's analog watches. Screw down locking crowns and traditionally operated water resistant crowns can not be operated under water.

Pushers

Digital and some analog chronograph diving watches - such as the IWC Aquatimer Minute Memory, the Omega Seamaster Professional, Breitling Avenger Seawolf Chronograph and Sinn U1000 - have specially-designed push pieces that can be operated at depth without allowing water to enter the case.

Helium release valve

Some diving watches intended for saturation diving at great depths are fitted with a helium or mixed breathing gas release or escape valve to prevent the crystal from being blown off by a pressure build up caused by helium that has seeped into the watch in helium enriched environments as the watch and diver adjust to normal atmospheric conditions. Other helium safe/for mixed-gas rated diving watches can withstand the helium used in certain diving situations by using gaskets that simply do not allow helium gas to enter the watch case in the first place.

Strap/bracelet

Most diving watches feature a rubber, silicone rubber or polyurethane strap or a metal bracelet of adequate length to facilitate wearing the watch over a diving suit sleeve. For this bracelets often have a (concealed) Divers extension deployment clasp by which the bracelet can be appropriately extended. One piece nylon fabric straps that slide under the watch case through both springbars are used to minimize the chance of losing the watch due to a springbar failure.

Legibility

The details and markers on the watch face and bezel have to be legible under water and in low light conditions. For easy legibility most diving watches have high contrasting, none cluttered dials and markers with a large, easily identifiable minute hand. The markers for 3, 6, 9 and (especially) 12 o'clock on the watch face and the zero marker on the bezel are usually conspicuously styled to prevent disorientation induced read out errors. For low light conditions luminous phosphorescent non-toxic strontium aluminate based lume pigments marketed under brand names like Super-LumiNova or NoctiLumina and tritium based self-powered lighting devices called "gaseous tritium light source" (GTLS) are used. On digital diving watches lightened displays are used.

Power reserve indicator

A diving watch with an electric powered movement must have an End Of Life (EOL) indicator, usually in the form of a two or four second jump of the second hand or a warning message on a digital display to safeguard against insufficient power reserve during underwater activities. Some electric and mechanical powered movement models have power reserve indicators that show the current power status of the watch. Mechanical movements should be wound or in case of automatic movements given enough motion before a dive.

Water resistance

The International Organization for Standardization issued a standard for water resistant watches which also prohibits the term waterproof to be used with watches, which many countries have adopted.

None of the tests defined by ISO 2281 for the Water Resistant mark are suitable to qualify a watch for scuba diving. Such watches are designed for everyday life and must be water resistant during exercises such as swimming. They can be worn in different temperature and pressure conditions but are under no circumstances designed for scuba diving.

The standards for diving watches are regulated by the ISO 6425 standard. Diver's watches must be water resistant at minimally 100 meters (330 ft) and are marked with the word "Diver's". The watches are tested in still water, thus a watch with a 200 meter rating will be water resistant if it is stationary and under 200 meters of still water.

Watches are classified by its degree of water resistance, which roughly translates to the following (1 meter = 3.3 feet):

Some watches use bar instead of meters, which may then be multiplied by 10 to be approximately equal to the rating based on meters. Therefore, a 20 bar watch is equivalent to a 200 meter watch. Some watches are rated in atmospheres (atm), which are roughly equivalent to bar.

Maintenance

Most manufacturers recommend diving watches have their seals changed and the watch pressure tested by an authorized service and repair facility every three years or so. 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. Divers also have to inspect their watch and wrist band for defects before every dive and especially in case it came into contact with gasoline or strong chemicals, powerful magnetic fields or was banged against something hard during use.