At the watchmaker's bench, "water resistance" isn't just the depth a watch can tolerate: it's a system of design choices that must all work together, or the watch is useless underwater. I've disassembled enough 1960s Submariners to know that even a single O-ring out of spec cancels out any figure engraved on the caseback.
The starting point is sealing. The case must resist hydrostatic pressure without deforming, and gaskets must prevent water from entering. So far, nothing mysterious. The subtler problem arises in saturation diving—the professional kind where divers live for days in a hyperbaric chamber breathing helium and oxygen mixtures. Helium has molecules small enough to seep through gaskets where water cannot pass. When pressure drops during decompression, the gas accumulated inside tries to escape: without a vent, it can pop the crystal or deform the case. The solution patented by Rolex in 1967 is the helium escape valve, the HEV, which opens automatically when internal pressure exceeds external pressure. Omega places it at 10 o'clock, Certina on the left flank, Tudor in compact form on the Pelagos. A different variant is the L-shaped gasket profile adopted by Seiko on certain Marinemaster models: it blocks helium mechanically, eliminating the valve.
The bezel is equally critical. On dive watches, the bezel must rotate only counterclockwise: if accidentally bumped during a dive, it can only reduce the calculated time, not increase it. An error that adds time is dangerous; one that reduces it forces the diver to ascend sooner, not to take more risk. The bidirectional bezel, which rotates both ways, exists but is conceptually unsuitable for professional diving use.
On the bezel, material affects long-term durability. The anodized aluminum segment scratches and fades; ceramic, like Rolex's Cerachrom or equivalent inserts from other manufacturers, maintains the legibility of the minute scale over time. The scale itself, when laser-worked in positive relief, resists abrasion better than an engraved and painted version.
For models with extreme resistance, beyond 2000 meters, the industry uses the informal definition "hors catégorie": specifications beyond recreational range, designed for scientific or military work. The EPSA Super Compressor case, widespread in the 1960s, used a spring caseback that increased sealing with depth. A clever idea for its time, now replaced by more precise solutions.
Sealing tests are divided into dry tests, using air pressure, and wet tests, with water immersion. They don't necessarily reveal the same defects: a watch that passes the dry test might fail the wet one. Both are necessary for serious certification.