Your watch’s water resistance is about hydrostatic pressure. Any movement in or under water increases this pressure. That first standing dive into the ocean or your backyard pool is nothing to you, but the pressure can blow out your watch in an instant.
Air and water pressure are everywhere around us. Even standing on a beach at sea level, the air blankets your body in 14.5 pounds of pressure per square inch. Years of evolution have made us impervious to the effects.
Things change underwater. Your body—and your watch—start coming under the effects of hydrostatic pressure, which is the force exerted on an object by liquid. It might not feel like much to you at certain depths, like a pool or a comfortable dip in the ocean. When you’re deeper than that, it’s a different story.
Plummet too quickly in a dive cage and you’ll burst your sinuses, give yourself a bloody nose, blow out the blood vessels around your eyes, get a nasty case of vertigo, and go deaf. That’s all assuming you live.
Get the lowdown on how deep down you can go with
101: The Real Take on Water Resistance Ratings
For every 33 feet you go down into the water, the pressure increases by 14.5 pounds per square inch. If you were a giant squid, which can swim at up to 7,000 feet below the water’s surface, you’d experience approximately 3,035 pounds per square inch of hydrostatic pressure.
But you’re not a giant squid, are you? The deepest man has gone without a craft surrounding him is 1,752 feet, relying on highly specialized gas mixtures and other dive equipment.
Your watch just probably isn’t going to make it that far. Unless you’re ready to drop beaucoup bucks, most water-resistant watches are good for swimming or snorkeling. Dive watches make it a little further, but to really go deep, you’ll have to invest in a SCUBA-certified watch.