How a watch keeps water out
A mechanical movement is destroyed by water, yet a good watch can be submerged repeatedly for years without harm. The thing standing between the two is not a single waterproof barrier but a system of seals — small elastomer gaskets compressed at every opening in the case, working together to keep the inside dry. Water resistance is an emergent property of this system, and because the system is made of parts that age and wear, it is the most underappreciated piece of engineering in the watch: invisible, consumable, and absolutely critical. Understanding it explains both how watches survive water and why their ability to do so is never permanent.
The gasket: a humble, critical part
The workhorse of the sealing system is the gasket — a ring of synthetic rubber (an O-ring) seated in a groove and compressed between two surfaces to fill the microscopic gap between them. There is a gasket at every opening: under the caseback, around the crystal, inside the crown tube, and around each chronograph pusher. When the case is closed, each gasket is squeezed into its groove, deforming to create a continuous waterproof seal. It is a simple, elegant idea borrowed from all of engineering, and it is the foundation of every water-resistant watch ever made. The number and quality of these gaskets, and the precision of the surfaces they seal against, determine how much pressure the watch survives.
The crown: the hardest opening to seal
Every opening is a vulnerability, but the crown is the hardest, because unlike the caseback or crystal it must move — it turns and pulls to wind and set the watch, so its seal cannot simply be clamped shut. A basic crown seals with a gasket inside its tube, adequate for splashes. For real water resistance, the screw-down crown threads onto the case tube and compresses its gaskets tight against the case, the way a screw-cap seals a bottle — the invention (Rolex's Oyster system of 1926) that made the genuinely waterproof watch possible. The trade-off is the small ritual it demands: the crown must be unscrewed to wind or set, and screwed back down afterward, and that final re-seating is the single most important habit in owning a water-resistant watch, because an unscrewed crown leaves the case's hardest-won seal wide open.
Chronograph pushers are sprung shafts that pass through the case, each needing its own gasket, which is why a genuinely water-resistant chronograph is a real engineering achievement and why so many warn against operating the pushers underwater — pressing a pusher down can break its seal under pressure. The most water-resistant chronographs use screw-down pushers, like screw-down crowns, that lock against accidental operation. Every function added to a watch is another hole to seal, which is the quiet reason complication and high water resistance pull against each other.
Why the seal is a maintenance item
The crucial consequence of building water resistance from rubber gaskets is that it degrades over time, independent of use. Rubber ages: it hardens, loses elasticity, takes a permanent set, and eventually stops filling the gap it was compressed into. This happens on a clock of roughly five to ten years whether the watch swims daily or sits in a drawer. A watch is therefore only as water-resistant as the current state of its oldest gasket — and the rating engraved on the case describes the watch as it left the factory, not as it is today. This is why gaskets are replaced at every service and why water resistance must be verified by a pressure test rather than assumed. "Water resistant to 100m" is a statement about the past; only a recent test speaks to the present.
Reading the sealing system
For an owner, understanding the seals turns abstract ratings into concrete habits: re-seat the crown religiously, never operate crown or pushers in water unless built for it, rinse off salt water (which attacks rubber), and pressure-test before trusting any watch — especially an older one — around water. For a collector, the sealing system explains why an unserviced vintage "diver" should be treated as not water-resistant at all, and why a service that includes fresh gaskets and a pressure test genuinely restores a capability that age had silently removed. The seals are where the watch's relationship with water actually lives, and that relationship is maintained, not given.
Water resistance is a system of small compressed rubber rings, anchored by the screw-down crown that tamed the hardest opening to seal — and built from rubber, it is always temporary. Understand the gaskets and the rituals they demand, and you understand both how a watch survives the water and why its survival must be renewed at every service rather than trusted to the number on the back.