The two terms are not interchangeable, and the difference is not marketing pedantry. "Waterproof" is a word watchmakers are no longer allowed to print on a dial in most markets. "Water-resistant to 200 metres" is a specific, testable claim. Understanding why the language changed, and what the depth number on your case back actually means, is the single most useful piece of dive-watch literacy a buyer can have.
Why "waterproof" disappeared
For the first half of the 20th century, watches were sold as "waterproof". The word implied a permanent, absolute seal. In practice no watch is permanently sealed, gaskets age, crowns wear, seals compress and relax with temperature. A watch that was genuinely sealed on the day it left the factory could leak two years later with no visible damage. Regulators in several markets, led by the United States Federal Trade Commission, concluded that "waterproof" was a claim no watchmaker could honestly keep, and it was phased out in favour of "water-resistant" with a stated depth. Today you will not find "waterproof" on a reputable Swiss dive watch. You will find a depth rating instead.
What the depth number actually means
A rating of "200 m" or "20 ATM" or "20 bar" is a pressure specification, not a diving instruction. It means the watch has passed a static pressure test equivalent to the water pressure at that depth. ATM (atmospheres) and bar are effectively interchangeable units of pressure: 1 ATM is roughly the pressure of 10 metres of seawater. So 20 ATM, 20 bar, and 200 m all describe the same test.
The catch is that the test is static. The watch is held at a fixed pressure in a lab. Real diving is dynamic: you move your arm, you enter the water at speed, the watch flexes against its gaskets. Water temperature changes the seal behaviour. For that reason the depth rating is best read as a headroom figure, not a limit you should approach. A watch rated to 200 m is comfortable at recreational scuba depths that rarely exceed 40 m, because the rating carries several multiples of margin over the actual pressure the watch will see.
A working guide to depth ratings
- 30 m (3 ATM). Splash resistance. Survives hand-washing and rain. Not for swimming, not for showering (hot water and steam are harder on gaskets than cold water).
- 50 m (5 ATM). A swimming watch. Fine in a pool or the sea at the surface. Not for any activity that drives water against the case at speed.
- 100 m (10 ATM). Snorkelling and surface water sports. The floor at which a watch starts to be genuinely capable around water, though not yet a dive watch by the modern standard.
- 200 m (20 ATM). A recreational dive watch. This is the practical standard for scuba. It is the water resistance of the modern Aquastar Deepstar and Model 60.
- 300 m (30 ATM). A working professional dive watch, with margin for repeated hard use.
- 500 m and above. Saturation-grade. The territory of the Aquastar Benthos 500, the first Swiss dive watch rated to 500 metres. More than a recreational diver will ever need, and built for commercial and technical work.
What keeps the water out
Three sealing points do the work: the case back, the crystal, and the crown. The case back seals with a compressed gasket or a screw-down interface. The crystal seals against a gasket in the bezel or case throat. The crown is the weak point, because it moves. A screw-down crown threads against a sealed tube and is the standard on any watch rated for real diving. A push-pull crown seals with friction gaskets alone and is more vulnerable. The single most common cause of a flooded dive watch is a crown left unscrewed, not a failed seal. Always check the crown is fully screwed home before the watch goes near water.
Why the rating is not permanent
Gaskets are the reason a depth rating is a snapshot, not a lifetime guarantee. Rubber and synthetic seals harden and lose elasticity with age, heat, and exposure to skin oils, soaps, and sunscreen. A watch rated to 200 m when new may not hold that pressure after five or ten years without a fresh set of gaskets. This is exactly why a dive watch needs a periodic pressure test and gasket replacement as part of a full service. On an Aquastar that interval is every 5 to 10 years, with no annual service needed even with daily diving. Keeping the seals fresh is what keeps the rating real. Read more on servicing a dive watch and on cleaning one without damaging the gaskets.
Frequently asked questions
What is the difference between waterproof and water-resistant?
"Waterproof" implies a permanent, absolute seal, which no watch can honestly guarantee because gaskets age and crowns wear. Regulators phased the word out in favour of "water-resistant" with a stated depth rating, which describes a specific, testable pressure the watch has passed rather than an absolute promise.
Does 200 m mean I can dive to 200 metres?
No, and you should not treat it as a limit to approach. The rating is a static pressure test, while real diving is dynamic. Read 200 m as a headroom figure: it gives several multiples of margin over the roughly 40 m a recreational scuba dive actually reaches, which is why 200 m is the practical standard for a recreational dive watch.
What do ATM and bar mean on a watch?
ATM (atmospheres) and bar are units of pressure, and for watch purposes they are interchangeable. One ATM is roughly the pressure of 10 metres of seawater, so 20 ATM, 20 bar, and 200 m all describe the same static pressure test.
Why does my dive watch need servicing to stay water-resistant?
The seals that keep water out, the case-back, crystal, and crown gaskets, harden and lose elasticity over time with heat, skin oils, and sunscreen. A depth rating is a snapshot of the watch when its seals are fresh, not a lifetime guarantee. A periodic pressure test and gasket replacement, every 5 to 10 years on an Aquastar, is what keeps the rating real.
What water resistance do I actually need?
For swimming, 100 m is comfortable. For recreational scuba, 200 m is the practical standard and gives ample margin. 300 m suits hard professional use, and 500 m and above is saturation-grade, built for commercial and technical diving beyond anything a recreational diver will reach.
Related reading: The six features that define a professional dive watch · Dive-watch cases and how they seal · The Aquastar Benthos 500 · Dive-watch glossary
