What is line cutter in diving?

A line cutter — a Trilobite, a Z-knife, a river-rescue hook — is a compact tool whose blade sits inside a hook or a slot, so it can never touch you. You catch the line in the hook and pull, and the blade parts it. Nothing about it is meant for stabbing, prying or cutting anything thicker than the slot, and that restriction is exactly the feature.

Why carry one instead of a knife?

Entanglement is the situation a cutting tool exists for: monofilament, gill net, an anchor line, your own SMB line wrapped around a valve. That is precisely what a hook cutter is good at, and it does it one-handed, by feel, with the blade shielded from your suit, your hoses and your fingers.

Advantages over a dive knife

• The blade cannot cut you. It is recessed. You can use it blind, behind your own head, in zero visibility, wearing thick gloves.
• Faster on thin line. One pull parts monofilament that a knife edge would slide off.
• Small, and mountable anywhere — a harness shoulder strap, a BCD pocket, a wrist. Reachable with either hand is the only mounting rule that matters.
• Cheap enough to carry two, one on each side, so an arm you cannot free is not the end of the story.
• Nothing to snag. No point, no exposed edge, no bulky sheath.

Disadvantages compared with a dive knife

• One job only. It will not cut a 20 mm mooring line, saw through kelp stems, pry anything, or hammer a signal on a tank.
• The line has to fit the slot. Thick or stiff rope will not seat in the hook.
• Blades are usually not resharpenable. A Trilobite blade gets replaced, not honed, and a dull one is worse than useless.
• Easy to lose. It is small, and small things get left in pockets and dropped.
• Anything needing force needs a knife. What is not a clean pull is out of its range.

What should you actually carry?

For most recreational diving a line cutter mounted where either hand reaches it is enough, and it beats a leg knife you cannot get to. For wreck, kelp, net-heavy or overhead diving, carry both: the cutter as the primary tool and a small knife or a pair of trauma shears as the backup. The real rule is two cutting tools in two different places — one tool on one side is one tangled arm away from having none.