HOW TO CHOOSE A DIVE KNIFE:
Use: 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.