How a dive bezel is used
A dive bezel tracks elapsed time underwater. Before descending, the diver rotates the bezel so the marker (the lume pip / triangle) lines up with the current minute hand position. After 20 minutes underwater, the minute hand is now pointing 20 minutes past the marker; the diver reads that directly off the bezel and knows their dive time. Combined with knowledge of their air consumption rate at that depth, they know how much bottom time they have left before they must surface.
Why unidirectional
If the bezel were bidirectional and got bumped during a dive (rocks, equipment, another diver), it could rotate either way. If it rotates so the marker is now behind the minute hand by 5 minutes, the diver reads "20 minutes" when they have actually been down for 25 minutes. Down 5 minutes of air on their mental budget, in a sport where running out of air is a fatal error. Unidirectional, counter-clockwise only: any accidental bump can only make the bezel show more elapsed time than has actually passed, putting the diver on the safe side of the calculation. Surfacing too early is fine; surfacing too late is not.
ISO 6425
The international dive-watch standard ISO 6425 mandates a unidirectional, counter-clockwise rotating bezel on any watch sold as a "Diver". The bezel must have at least one marker visible underwater (luminous), must show every 5-minute segment, and at minimum the first 15-20 minutes must be marked at 1-minute intervals (the most critical decompression window). This is why a Rolex Submariner, Omega Seamaster 300, or Tudor Black Bay all click only counter-clockwise. Vintage divers (pre-1980, before ISO 6425 was finalised in 1996) sometimes had bidirectional bezels; collectors look at this as a warning that the watch is aesthetic rather than dive-rated.
When bidirectional is correct
Other complications use bidirectional bezels because there is no safety case for direction lock-in. GMT bezels: rotate to align the 24-hour scale with a second time zone; you may need to go either way. World-time bezels: same, but with city names. Compass bezels: rotate freely to track north. Tachymeter / slide-rule bezels: pure scales, no time-tracking. Pilot watches: usually bidirectional, used for general timing and navigation calculations. The Rolex GMT-Master II bezel is bidirectional; the Submariner bezel is unidirectional. Same brand, different complication, different safety logic.
What "click stops" mean
A unidirectional bezel typically has 120 click stops (every 30 seconds) or 60 click stops (every minute). More clicks = finer adjustment, but each click is shallower and easier to miss. Premium dive watches usually run 120-click bezels with strong, audible detents (Tudor, Omega, Rolex). Cheaper watches sometimes feel mushy or rotate too easily. A bezel that does not click solidly is a sign of a poorly-engineered case (or a worn-out one in need of service).
Buying notes
On the used market, a dive watch with a bidirectional bezel is either pre-ISO-6425 vintage or a non-dive sport watch. Both are fine, just know what you are buying. Older Rolex Submariners (pre-1959 ref. 6204/6205) had bidirectional bezels; they are still legal dive watches by 1950s standards but would not be ISO 6425 compliant today. On a brand-new dive-rated watch, a non-unidirectional bezel is a red flag indicating either a counterfeit or a dressy "diver-style" watch that should not actually be taken below 30m. For more on dive-watch standards see our water resistance guide.