One could also say don't show up at a knife fight with scissors!
There are essentially two sorts of divers these days:
1) The techical sort, no, not the tech diver exclusively but the sort that is into gear and technology and all the gizmos and dives so he/she can justify buying more stuff to dive more with and are heavily peer influenced. These guys have scissors, small blunt tippped "tools" and usually multiple versions of each strategically placed about their person.
2) The pretty fishy sort, a quasi sugary PC mentality that mind melds with the environment. These types carry no knife at all even with a blunt tip, to dangerous.
Once upon a time back in the Scubazoic Era there was a thrid type, the Adventuer, a romantic sort. Behind every ledge was a bad guy, under every rock a giant octapus, in the depths were unknown monsters and every dive was a wonderland of danger both real and imagined, a colorful and fanciful place to lose oneself in for a lifetime. This type is known to carry a big knife, though essentilly extinct a few specimens persist.
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