When we first started diving it was cool to carry the longest and biggest knife you could strap on your leg like James Bond in Thunderball.  We were all thinking of protection, as if a big knife would keep you from getting bitten by something large with a mouth full of razor sharp teeth.  Now I carry a 4″ blunt tip serrated knife.

Now divers carry knives for different reasons.  Mostly to untangle themselves or others from fishing lines and types of entanglements.  I also carry a utility scissors I used as medic.  These will cut through almost anything and are made of rust resistant materials.  Today many divers carry their knives attached to their BCD.  The thing to remember is, you need to place it in a spot where you can EASILY reach it if you do get tangled. If you keep your knife in good condition (clean it after each dive and oil it), it will last a very long time.

Here are some sites to help you compare different types of dive knives:

http://www.scubadiving.com/gear/accessories/GR_acc_15_sensible_dive_knives

http://www.scuba.com/scuba-gear/Knives.html

http://www.scubatoys.com/store/Scuba_Knives.asp

http://www.amazon.com/s/ref=sr_pg_1?ie=UTF8&rs=&keywords=dive%20knife&rh=i%3Aaps%2Ck%3Adive%20knife&page=1

http://www.nextag.com/titanium-dive-knife/search-html