Why is that vital? If a unidirectional bezel is knocked and accidentally moved with a dive, it can only display an extended underwater time. That means the wearer will overestimate rather then underestimate immersion time, As well as in conditions that involve decompression stops, this can be a make-it-or-crack-it issue https://www.watchesworld.com/frozen-in-time-luxury-watches-for-winter/