Water resistance is not a binary feature: it's a system. A watch is not simply "waterproof" or "not waterproof" — it has a set of physical barriers working together, each of which can fail independently of the others. From the bench I've seen perfectly dry movements inside cases with deteriorated caseback gaskets, and vice versa. Those who don't understand this buy used watches and take them underwater without preliminary testing.
The three infiltration pathways are always the same: the crown, the caseback, and the crystal. The crown is historically the most critical point because it moves, wears, and is operated frequently. The most common solution is the screw-down crown, which compresses one or more elastomer gaskets by screwing against the case tube. Rolex made the Triplock system famous, with three concentric gaskets; other manufacturers, like Sinn with the D3, have eliminated traditional screw pushers by replacing them with integrated solutions. The practical result is the same: zero room for water. The Fifty Fathoms MIL-SPEC from the 1950s went beyond physical sealing and mounted a water-resistance indicator on the dial — a colored disc that signaled to the diver whether integrity was still guaranteed before diving.
The screw-down caseback is the second barrier. Here the O-ring gasket works through circular compression: the tighter the caseback, the more compressed the gasket, the more effective the seal. When rubber ages, hardens, or becomes permanently crushed, the seal fails even without anything appearing broken. This is why every overhaul on a dive watch should include systematic gasket replacement, not just visual inspection.
The crystal is the third variable. In the most elaborate cases — like the Griffes on the Omega Constellation from 1982 — the crystal seal is mechanically assured by four screwed elements that tighten sapphire and gasket against the case. Bulova patented a "capsule-type" system in the 1950s where crystal, dial, and movement formed a sealed unit inserted into an external bezel. Citizen introduced Parawater technology in 1959 with O-rings at all access points. Different solutions, same principle: isolate the movement from the external environment.
Water resistance declared in ATM or meters is always measured under static conditions and at constant temperature. Moving water, rapid thermal variations, and gasket wear over time lower that threshold. The pressure test — performed in the laboratory with calibrated instrumentation — is the only way to verify the actual condition of a used watch before submerging it.