How do I operate and how much should I tighten the screw-down crown?

Many watches that are highly water-resistant have screw-down crowns. Essentially, the crown of the watch (the knob on the side used to wind and set the watch) screws in place like a tiny bottle cap to make sure that part of the watch is extra water-resistant.

To open the crown, simply turn the crown anti-clockwise to unscrew it (takes about 4 or 5 turns). To close it back, push it in while turning clockwise. Do not close it real tight, just with mildly firm finger pressure. It is the threading that provides water-tightness, so over tightening does nothing to increase the water protection and can overstress the metal, eventually damaging it and ruining the water protection of the crown.

A few diving watches also have a "helium relief valve." If yours does, check that it is closed similarly.

Once the crown is unscrewed and in the "neutral" position, the crown operates like any normal watch.