Water Resistance, What Does It Mean?
Many manufacturers stamp the back, and tout in advertisements, that a particular watch is 'water resistant', followed by a number. But what does that mean?
In years past a watch would be labeled 'water resistant', or even 'water proof'. But the distinction was essentially arbitrary and was eventually dropped. The label has been replaced by a measurement of a watch's resistance to infiltration of water under pressure.
Water exerts pressure on a watch and when it seeps into the interior can cause damage by rusting parts, slowing down a mechanical mechanism or short circuiting electronics. To prevent that from happening manufacturers design cases to resist that pressure and keep water out.
Key to that goal is the use of precision parts and miniature seals.
Watches today are created with extremely precise tools that measure part dimensions very accurately. That way when the watch is assembled everything fits exactly. No gaps, no place for water to come in. But even the most precise tooling in use can't provide a close enough fit to keep water under pressure from squeezing through a tiny gap.
To do so is possible but would require machinery and techniques that aren't cost effective. So, they employ gaskets, sealant and techniques to block any gaps that might exist. Tiny o-rings are used to seal point of entry of the stem into the case, around the crystal and often the back. Battery compartments are similarly sealed. Creating screw-on backs, rather than snap-on ones, helps, too.
The whole system contains sealant in a dozen places. That allows manufacturers to achieve the ratings you sometimes see stamped on the back, or listed in ads. Common ratings, for example are 30M, 50M, 100M or higher.
The 'M' stands for meter and the number indicates that a watch will withstand water pressure up to that found at, say, 30M of depth (equivalent to 3ATM, or three atmospheres). But the ratings, unfortunately, don't really reflect real-world conditions. The measurements are made under very carefully controlled lab conditions, using static pressure.
Water exerts a force over the area of a stationary watch from all directions. That's static pressure. But any watch that's worn is almost always in motion, if the wearer is (even partly) underwater. Whether in the swimming pool, or just washing the dishes or taking a shower, your arm is often in motion. That increases the pressure substantially.
So, even though a watch may be rated safe to wear up to 100M of depth in the water, in fact, you wouldn't want to do more than go snorkeling near the surface with one. A 50M watch is usually safe to shower with or wash a dish in the sink. Pressure higher than that can cause leakage.
Since manufacturers know this, and they don't want to get sued, they accompany the ratings with recommendations. They'll state openly that a 100M watch should not be used for scuba diving to a depth of 100M. The wearer should limit use to casual swimming near the surface.
If you really want a 'water proof' watch, or one safe from leakage at more than a few meters under the water, look into a genuine diving watch. For casual showering, and other everyday activities a standard 50M watch will do just fine, provided none of the seals or the crystal have been compromised.