Why the depth ratings lie
A 30-metre water-resistance rating doesn't mean you can take the watch 30 metres deep. The rating is a static pressure test: the watch sits in a pressure chamber and is exposed to the equivalent water pressure for a few minutes, in a lab, at room temperature, with the gaskets fresh. Real-world wear adds hand movement (which creates dynamic pressure spikes), temperature cycling (hot tub then cold pool), gasket aging, and crown actuation. The industry rule of thumb is to derate the marked rating by half or more for safe real-world use. So a "30m" watch is splash-proof at best.
What each rating means in practice
30m / 3 ATM: hand-washing, light rain, accidental splashes. Do NOT swim, shower, or hot-tub. Most dress watches sit at this rating because they're not designed to be wet. Patek Calatrava, Cartier Tank, most non-water-rated vintage.
50m / 5 ATM: swimming in a pool. Avoid jumping into water (diving impact creates pressure spikes that exceed the rating). Do not shower (hot water expands the gaskets, soap degrades them). Most everyday "water resistant" mid-tier watches.
100m / 10 ATM: swimming, snorkelling, casual water activities. Modern ISO 22810 certifies this for "everyday water-related activities". Most luxury sports watches not marked as divers (Royal Oak, Nautilus, Reverso Tribute Tide) sit at 50-100m.
200m / 20 ATM: recreational scuba, snorkelling, surface water sports. Crosses the threshold for screwed-down crown construction. Tudor Black Bay, Omega Aqua Terra, Seiko Prospex.
300m+ / 30 ATM: ISO 6425 dive-watch territory. Adds unidirectional rotating bezel for elapsed-time tracking, large luminous indices, salt-water resistance, helium escape valve on some references. Rolex Submariner, Blancpain Fifty Fathoms, Doxa Sub 300, Omega Diver 300M / Planet Ocean.
1000m+ / 100 ATM: pro-saturation diving territory. Helium escape valve mandatory. Rolex Sea-Dweller (1,220m), Deepsea (3,900m), Omega Ultra Deep (15,000m). Real-world dive use never gets near these depths; the ratings are engineering flexes.
The crown is the weakest point
Water enters through the crown first, then the case-back, then the crystal/bezel seal. A screw-down crown adds 2-3 ATM of pressure resistance and is mandatory above 100m. A push-pull crown is acceptable up to 50-100m. NEVER operate the crown underwater; even on screw-down designs, the crown is not actively sealed when you open it for time-setting.
Real-world rules
Get a pressure test annually if you swim with the watch. Most service centres do this for $20-50; the watch is sealed in the lab and pressure-cycled. Expect gasket replacement every 3-5 years even on unworn watches; rubber dries out. Do NOT shower in any watch; the temperature shift + soap is harder on gaskets than diving. Hot-tubs are the worst (heat + chemicals + pressure cycling). Saltwater requires fresh-water rinse afterward; salt accumulates in the case-back groove and crown.
Bottom line for buyers
For a single watch you wear daily and want to swim with, buy at least 100m rated. For active swimming or diving, 200m+. For dress watches you keep dry, 30m is fine. Treat the marked rating as the watch's upper bound, not its operating spec. See wiki: water resistance and ISO 6425 dive-watch standard.