Water-resistance ratings confuse almost everyone, because the numbers do NOT mean the depth you can safely go. They come from a static pressure test in a lab, not real swimming or diving.
30m / 3 ATM: splashes, rain and hand-washing only — not swimming. 50m / 5 ATM: light swimming and showering, no diving. 100m / 10 ATM: swimming and snorkelling. 200m / 20 ATM and above: suitable for scuba diving.
The test applies still pressure to a still watch. Real water adds sudden force when your arm moves, when you dive in, or under a tap — which can briefly exceed the rating. That is why a '30m' watch should never actually go swimming.
Always push the crown fully in (and screw it down if it screws); never operate the crown or pushers underwater; rinse with fresh water after the sea; and remember the rubber gaskets that keep water out age over time, so a watch needs its seals checked every few years to keep its rating.