How exactly is water resistance rated and why?
Different manufacturers use different measurements to display their water resistance, but it all comes down to the same thing. The use of the term 'waterproof' was prohibited in 1990 by the International Organisation for Standardisation as described in ISO 2281 because it was misleading. Effectively saying that a watch would never leak under any circumstances.
'Depth' ratings on watches do not indicate how deep you can go before it leaks. They actually indicate equivalent pressure. A watch rated for 30 meters sounds okay for swimming and even diving, but that is incorrect. It actually means that it's rated for the pressure normally encountered 30 meters under water in static conditions.
Think you won't encounter that in everyday life? Just putting your watch under a tap running with decent force is enough to exceed that pressure.
Watches with any of these markings on them are rated for splashing and getting caught in the rain.
50 meter rated watches are fine for still water swimming such as a pool.
100 meter watches are fine to wear in surf, snorkelling and shallow diving.
200 is fine for recreational diving and active water sports.
Extreme depth diving. These watches can handle as deep as you can, but need specialist resealing to maintain it.
A fairly costly mistake some watch owners make is that it's fine to expose your watch to hot water such as a shower.
Watches are not necessarily designed or rated for hot water
As watches are exposed to hot water the air inside the case expands. Due to being compressed, the air pressure inside the case climbs, and the sealing system built into the watch allows the increased pressure to escape in order to equalise the pressure.
The problem however, is when the watch cools. The pressure inside the watch decreases, which equalises the pressure by allowing air from outside to enter the watch.
When this occurs, the air picks up moisture around the seals drawing it straight into the case.
It only takes a single drop to destroy a watch movement
If a watch is worn in water, we advise you get the watch resealed and pressure tested every time it's opened, such as for a battery change, to ensure it can properly resist the entry of moisture.
Screw-in crowns must be locked down before exposing the watch to water. If the crown isn't properly closed, then the seals will be out of position and allow the free flow of water into the watch
Solar, automatic and kinetic watches
Seeing these watches do not come in for periodic servicing or battery replacement, the seals may deteriorate, fail and let water into the case before the watch needs attention.
As recommended by watch manufacturers, these watches should have the seals checked periodically to maintain their water resistance.