The knife is not used in diving as a weapon but as a safety instrument that can be used to cut a cable or a net, to hit the bottle to get the attention of the companion, etc.
ITS USE SHOULD NEVER CAUSE DAMAGE TO MARINE LIFE
In view of the various purposes for which it is intended, the knife blade must be strong and stainless steel. The handle must be robust, suitable for blade size and anatomical, so as to ensure safety when the knife is in use. The knife can be one or two-edged (dagger). The option for a knife with a one edge and a thick side is preferable, because this type of knife is more versatile.
However, whichever model you choose, one side must have a long, sharp serration, capable of cutting a cable. The edge should also be sharp, so as to easily cut fishing lines or nets.
Usually the knife sheath is attached to the leg or diving vest, allowing easy and convenient access.
ALTERNATIVE OR COMPLEMENT
Alternatively or complement the knife, there are also diving shears, designed to cut nets and cables of small gauge. They are very effective instruments, with the advantage of working well with only one hand available.
Although made of stainless material, with time and poor maintenance these instruments begin to acquire rust. This degradation can be avoided if at the end of each dive these instruments are washed with fresh water and thoroughly dried before being stored.
The knife, or dagger, should always be stored inside the sheath to avoid accidental cuts.