Dive Knives
Purpose
A dive knife is really a tool, that you can use underwater in an emergency or at the surface. Underwater, you can use it to cut yourself or your dive buddy out of an entanglement scenario, remove unwanted fishing line or netting from the bottom, cut through kelp or other tangling items. At the surface, a dive knife is handy to cut dive reel line, tighten a loose bolt, or make a sandwich during the surface interval.
Features
Size
If you are diving in warm water, a smaller more compact knife will be acceptable. For cold water diving, keep in mind you will have thick neoprene gloves on so a larger knife handle may be easier to operate.
Attachment Point
Smaller knives may be attached directly to your BCD, inflator hose, gauge hose, or stored in a pocket. A larger knife will likely be attached with straps to your leg and may be too large or cumbersome to be attached to your BCD.
Blade Type
A pointed tip may be helpful for those of you who spearfish or harvest (legally of course). Blunt tips can make a handy implement such as a screwdriver or a prying device.
Metal Type
High quality stainless steel such as 400 grade steel will keep a sharp edge longer but will require more maintenance, such as regular rinsing. Titanium is less susceptible to corrision, is lighter weight, but costs 30-40% more than stainless steel. With either material, look for a blade that extends through the handle and can be removed for proper cleaning.
Other Options
Sea snips (shears) or line cutters. Both make a great backup to a primary knife.
Care & Maintenance
After diving, always give your dive knife a good thorough rinse with fresh, clean water to remove any salt or chlorine. Stainless steel will rust over time if not properly taken care of. Some knives will allow you to remove the blade from the handle which allows for a much more thorough cleaning. Leave the blade to dry completely before placing it back into the sheath. Rub a little silicone or oil into the knife before storing for prolonged period of time. It may need to be sharpened before storage if you have been using it a lot.