Quick answer: This is a dive knife or cutting tool from Scubapro.
Every diver should carry something that cuts line. Mount it where you can reach it easily with either hand, which for many bigger divers means the BCD or harness rather than the leg.
Check the blade style, sheath and mounting options, and rinse it with fresh water after every dive. Read Best Scuba Knife for Safety. More options: Cutting Tools & Knives.