Companies which produce wristwatches rate their product for it's ability to stay dry underwater. What does that rating really mean? Watches have carried different monikers indicating how they will do in water. These phrases never had any science behind them aren't used anymore. Now a number reflecting depth of use has been adopted.
As some moisture might get through it can incite many issues with the works on the inside, making it impossible to give accurate time. To prevent that from happening manufacturers design cases to resist that pressure and keep water out. Important in that intent is modern engineering and machinery.
No gaps, no place for water to come in. But even the most precise tooling in use ca n't provide a close enough fit to keep water under pressure from squeezing through a tiny gap.
A company could decide to make a watch of this kind, they could never sell them for a price buyers would pay. Watch makers, like Rolex, choose to seal the watch case by other means. 
It is then possible to state how deep you can go with a particular watch. Common ratings, for example are 30M, 50M, 100M or higher.
They are rated on conditions not apparent in a real underwater situation. The measurements are made under very carefully controlled lab conditions, using static pressure.
Your watch if submerged in water is likely to be involved in action. Making the rating on the watch less than stated.
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. Pressure higher than that can cause leakage.
Of course companies are aware and don't want trouble, they have added statements to limit their liability. In the manual the rating might be explained further in a way that says not to use it as deep as it says. Wear the high end watch you bought in the bathtub but not diving.
For people who dive and want a watch for that purpose you have many options.
