Although the popular image of a dive knife may be to fend off sharks or other hazardous marine life, nothing could be further than the truth. The real reason that divers carry knives is simply for safety, in case of becoming caught or tangled in some obstacle such as discarded fishing net, or for cutting through monofilament. Secondary uses can be used as a rough rule - as objects are magnified in the water it is sometimes useful to be able to compare them with something with known dimensions, and a dive knife performs this task well.

The considerations for a dive knife are simple. They should be worn in a place that they can easily be accessed and easily withdrawn, but should hold firmly in their sheath when not in use. Many divers wear a knife on the inside of their calf, other options include the BCD, or the BCD inflator hose. Specialist divers such as wreck divers sometimes wear two or three knives. This is simply a question of redundancy - a diver caught in the confines of a wreck may not be able to reach a calf mounted knife, but could reach a hose mounted one, or vice versa.
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Reviews

The Wenoka Big Squeeze dive knife (shown above) is designed by legendary knife designer Blackie Collins and it's simply the best dive knife we've found on the market. The knife is held firmly in its sheath by a couple of spring loaded hooks and to remove it the diver simply takes hold of the handle and squeezes either side before withdrawing. This makes the Big Squeeze simple to use, even with thick gloves, while extremely secure so you can be sure it'll be there when you need it. The review model is stainless steel and although used on about 20 dives over three months still shows no signs of corrosion. The knife is also available in titanium. The knife can be attached to some models of SeaQuest/Aqualung BCDs, which have riveted holes located above the left hand pocket through which "barnacle bolts" (available separately) can be screwed to the sheath.

The Omer Titanium Thunder (left) is a great looking, well balanced dive. The titanium cutting blade has a good sharp edge and it also has a monofilment cutter and serrated edge. The problem with it turned out to be the sheath. The knife is held in by a simple latch, but the review model was lost during a dive. The design of the sheath appears to have changed since, so perhaps it retains the knife better than before.