Others dive without any cutting tools, which to my mind is a lapse in judgment when it comes to personal safety. Entanglement involving discarded netting or fishing-lines is always a possibility, even in supposed no-fishing zones and protected marine parks.
We know that wrecks found off the UK coast are notoriously precarious places to dive because most are festooned with lost nets, monofilament and braided lines. This is where every diver needs to carry a tool capable of effortlessly cutting through any material likely to be encountered.

The Design
The Aqua Lung Argonaut’s blade, hilt, handle and butt are fashioned from a single section of 4mm thick titanium alloy. This material is lightweight but extremely tough and corrosion-resistant, so is suitable for tools used in a marine environment.
The metalwork is coated in black electro-phoretically deposited paint (EDP) with laser-etched livery. The full blade length has die-cut serrations on one side and a ground-smooth edge on the other to provide two “cutting platforms”, the butt forming a tactical striking tip.
The Argonaut comes with either a spearhead-shaped point or a blunt chisel tip – the latter was the one tested. The handle is double-wrapped with 5mm paracord and tied off with a 12cm leash. The wrap finishes 12mm from the hilt to provide a spot for the forefinger to rest and give unhindered entry to the sheath.
The unsheathed knife weighs in at a measly 106g and is 24.8cm long, the blade taking up exactly half of this length.
The sheath is made of 2mm-thick Kydex plastic thermoformed around the knife’s profile and fitted with seven grommets and four slots to provide BC, thigh, calf or forearm mounting options plus an optional neoprene leg-strap (pictured). It is finished with laser-etched livery and comes with twin rubber leg-straps. There is also a single drain-hole at the base of the knife slot.

The Test
I set about slicing or sawing my way through an array of ropes and monofilament lines of various materials, diameters and breaking strains, as well as a selection of the modern fishing braids that have become so popular with fresh- and saltwater anglers. I also included some 2mm steel cable and electrical wire.
To test the Argonaut’s suitability as a cutting implement in real-world conditions I formed handheld loops, as this is the way I’d expect an entangled diver to deal with entanglement.
The tough steel cable took just over 20 seconds to saw through using the serrated edge, but this knife cut through everything else with ease.
Surprisingly, the braid parted the instant it came into contact with either side of the blade.
I say “surprisingly” because this line is almost impossible to break with bare hands, and will cut through flesh like a surgeon's scalpel if pressure is applied.
It's the potential of encountering this sort of hateful material under water that makes carrying a knife so important – it's so fine for its given breaking strain that it’s almost invisible, but it’s no match for a sharp blade.
As an emergency tool, a knife must be secured where it can be easily reached.
The Argonaut sheath proved versatile, and I could use the slots and Velcro straps to attach it to my BC inflator-hose or cable-tie it to my BC waist.
I have never been a fan of calf-mounted sheaths, because rubber leg-straps tend to loosen as depth and pressure increase, and compressing exposure suits can allow a sheath to slip and end up flopping around my ankle. However, Aqua Lung’s leg-strap was different.
Made from 5mm neoprene with a grippy “skin-out” back surface to provide traction, it has twin adjustable 25mm webbing straps with Fastex buckles and a double-layer hook-and-loop attachment system.
Mounting it at my calf or thigh gave me a feeling of security I haven't experienced before, and it stayed put whatever my depth and held the sheath firmly in position.
Strap adjustment was easy, as was donning and doffing.

Conclusion
I chose the “Blunt” version of the Argonaut to test for several reasons. Firstly I’m a clumsy oaf, and was sure to have stabbed myself or punctured my drysuit or BC air-cell with a pointy-end knife during the test process.
Secondly, I didn’t want to be doing an impression of a macho serial killer or off-duty Special Forces operative to anyone who cared to notice.
And most importantly, the chisel tip is actually a very useful additional tool.
The Argonaut was an efficient cutting instrument, slicing through braided fishing-lines in an instant with the smooth side but also sawing easily through everything else, including rope up to 30mm diameter and steel cable with the serrated edge.
The simple paracord-wrapped handle provided superb grip as well as a secure wrist lanyard, and the sheath held the knife solidly in place. Add in the advantages of using titanium alloy and it left me thinking there probably isn’t a better mid-sized knife out there, or one as good-looking.

SPECS
PRICES: Knife £104, Neoprene Leg Strap £15.50
LENGTH: 24.8cm (blade 12.4cm)
WEIGHT: With sheath 146g
CONTACT: www.aqualung.com/uk
DIVER GUIDE: 10/10