Dive Knives

Carrying a dive knife is basic dive safety. Dive knives should not used as weapons, they're used to free yourself from entanglement, particularly from fishing line and nets. Also Divers can get caught in their own dive lines and spear gun lines. Many divers carry more than one dive knife for added safety and it's advisable to wear one on the BCD and one strapped to a leg.

Resource from PADI
How to choose Scuba Dive Knife