In spite of what some people may say, you are free to carry just about any kind of knife if you want to. However, most would suggest that a big 'ole nasty Rambo knife is not a good idea, even though many guys are instinctively drawn to the big steel.
Just a few things to consider:
1) That's a big chunk 'o steel. Strap it to one leg, and you are heavy on that side.
2) A great big pointed thingy is just looking for other things to poke into or through. For every one thing that you really really want to poke, you will probably accidentally poke at least 5 others that you didn't want to have holes in.
3) A big sharp edged cutting tool is just looking for things to slice through. . . etc.
4) In some places you may be asked to not carry a knife that large.
5) In some places (ScubaBoard included) you may be harrassed or teased for carrying a Rambo knife.
6) In many places you will probably be viewed in a negative light for carrying a Rambo knife. I know; who cares what other people think? It might make people avoid you in situations where you could use a buddy. Of course the other side of the coin is that you might attract some divers to you because of your cool weapon.
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Think about what you carry a "knife" for. Of course there are many reasons, but the primary reason would be to cut entanglements. Many people would think that a line cutter and/or a pair of shears ( Such as those shown in response #4) is/are a much better safer, saner way of getting the security you need. If you do carry a knife (and many divers do) you might want to consider something a little less massive, and certainly consider a blunt tip version that is much safer.
However, if you really have your heart set on strapping a Navy Seal style pig sticker to your body, then go for it, but be prepared for the consequences.
My two cents, and only because you asked.
Wristshot