What is dive knife in diving?

A dive knife is a fixed or folding blade in stainless steel or titanium, usually with a plain edge on one side, a serrated edge on the other, and often a notched line cutter in the spine. Its job is everything sharp on a dive: freeing yourself or a buddy from rope and netting, cutting away a fouled line, prying, digging, and — with the butt — making noise on a tank to get someone's attention.

Do you still need one?

Less than the old catalogues suggested. The big leg knife was standard for decades and has mostly disappeared, because the thing it was carried for — entanglement — is better handled by a line cutter, and because a large blade strapped to a calf is both hard to reach and easy to snag. What survives is the argument that a knife does what a cutter cannot: thick rope, kelp, and anything needing force rather than a clean pull.

Where should it live?

Where either hand can reach it while you are tangled. In practice that means the inside of a forearm, a harness shoulder strap, a waist belt or a BCD pocket. A calf sheath is a reasonable place for a second knife and a bad place for your only one: if the tangle is on your arms, your leg is unreachable.

Fixed blade or folding?

• Fixed blade — nothing to open and nothing to fail, always ready. Wants a sheath that holds it under load and does not go brittle in sunlight.
• Folding — packs smaller and cannot fall out of a sheath, but needs two working hands to open, which is the exact resource an entanglement takes away.

Blunt tip or pointed?

Blunt is the sensible default. A pointed tip buys you a puncture and a prying lever, and costs you a hole in a drysuit, a BCD or a buddy. Choose pointed only for a real, specific use.

Stainless or titanium?

• Stainless (420, 304, H1) — takes a keener edge and is cheap to replace, but even "stainless" rusts in salt water without a rinse and a light oil.
• Titanium — will not corrode and needs no maintenance, holds an edge less well, and costs more. Worth it for a knife that lives on a harness and gets checked more often than it gets used.

Rinse it in fresh water after every salt dive, dry it, and check both the edge and the sheath retention before you rely on it. A knife that will not come out of its sheath, or will not cut when it does, is dead weight.