3 Functions of a Watch Bezel You May Not Know | WatchMaestro 

The bezel on the Sub nothing to do with how much air you have. It’s about bottom time; how much time you can safely stay at depth before you need to head up. If you stay past that time you would need increase time at decompression stops.

Great job - nicer watches. For ‘desk divers’ bosses will say ‘you have 5 minutes’ - for a presentation, pitch, explanation, etc. His time is valuable 😀

You're starting off clueless already. The bezel on the DateJust isn't just for aesthetic reasons. It's there to hold the crystal in place. Divers don't breathe oxygen. They breathe air (or nitrox, trimix, heliox, etc.) Oxygen is toxic below 6 meters or 20 feet. You use the bezel as a bottom timer. It has nothing to do with gas consumption. It has everything to do with no decompression limits. The tachymeter bezel isn't for timing laps. That's what the chronograph is for. You use the tachymeter scale to time a known distance, like 10 hectometer signs, and you can read your average speed on the tachymeter bezel. Circuits aren't exactly 1 km, 2km, 5km, or 10km. They're 3.337 km, or 5.891 km, or 5.809 km, or 20.830 km. A tachymeter scale is completely useless for a full lap. The seconds subdial is not part of the chronometer. That's the running seconds of the regular watch. Your explanation of the GMT bezel applies to the GMT-Master. The GMT-Master II has an extra trick up its sleeve. You set the GMT hand to your home time, your hour hand to the local time, and set the bezel to the time offset to your home time for a third time zone. Also, how can you not include the Sky-Dweller? That's the most impressive bezel of any Rolex watch.