If one is diving in less benign places, such as kelp, okay, a knife is a must. For the rest of us, it is unlikely to ever see use.
WHAAAAATTT!?!?!?!
WHAT ABOUT MY INEVITABLE BATTLE WITH THE KRAKEN??!?!?!?!??!
I won't say which country it was, but one of them, every single guy had a BFK strapped to his lower leg. Sorry mates, but anything that comes after you, is going to be much faster than your ability to even begin to reach for your BFK.