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Timing with a divers watch is more about decompression stops as the more time spent at a certain depth = longer decompression stops on the way back up.
Depth and tank pressure gauges are attached to regs. Most diving is no-decomp recreational. The bezels are used to track elapsed dive time and safety stop time. You need both so you can work out your pressure group, so you can work on surface intervals so you know when you can go back down for the next one. It's all on a table but we dive with nitrous so it's all dive computer anyway.
Must be dive tank minutes.