A dive watch is not simply a waterproof watch: it is a precise technical category, defined by the ISO 6425 standard, which imposes verifiable and repeatable minimum requirements. The difference from an ordinary water-resistant watch lies in the testing methodology — static and dynamic pressure, shock resistance, legibility in low-visibility conditions — and in the presence of functional elements that a generic wristwatch does not need to have.
The most visible requirement is the unidirectional rotating bezel. It rotates only counterclockwise: if accidentally struck during a dive, it can only indicate a shorter remaining time, never longer. It is a passive safety principle, not an aesthetic gimmick. Models with ceramic bezels add scratch and marine corrosion resistance, maintaining scale legibility over time. There is also the internal bezel solution, controlled by a separate crown: the choice of the C65 Super Compressor Elite, which revives a system patented by EPSA in the 1950s in which hydrostatic pressure itself improves the seal of the gaskets the deeper you descend.
The screw-down crown is another structural element: it closes with a thread against the case, compressing a rubber gasket. Without this mechanism, any pressure variation during ascent can force water into the gap. On watches intended for technical diving or saturation — where the diver lives in a hyperbaric chamber for days — a helium release valve is added. Helium, used in breathing mixtures at great depths, is small enough to penetrate even through intact gaskets. Without the valve, the pressure accumulated during decompression could blow off the crystal. The valve can be manual, as on the original Sea-Dweller, or automatic; the standard position is at 9 o'clock or 10 o'clock, but each manufacturer has its own choices.
The distinction between skin diver and scuba diver, in the technical jargon of the 1960s and 1970s, had practical significance: the former was thinner, designed for the freediver or swimmer with light equipment; the latter was built to withstand the pressures of the self-contained underwater breathing apparatus and often certified for professional use. Today the distinction is almost purely historical and collectible, but it still guides the interpretation of certain design choices in vintage watches.
The luminosity of the indices — today entrusted almost universally to Super-LumiNova — is not a decorative detail: underwater, sunlight attenuates rapidly with depth, and hour legibility is a matter of orientation and safety. The same applies to the decompression scale, which on some models from the 1960s and 1970s was engraved on the caseback or integrated into the dial as a mnemonic aid for decompression stops.