I assume you mean in reference to airline travel and anti-terrorism? As far as I know, there are no restrictions on how long a blade you can put into checked luggage on airlines; the fact that it's in with dive gear would obviously help your case.
Another issue is whether ecology laws in a particular country or area will allow divers to carry a dive knife, or for that matter any cutting tool. Many (all?) areas in the Egyptian Red Sea prohibit knives; Mania says that even applies to cutting tools like shears or z-knives... and that the law is strictly enforced.
I just got back from two weeks at Makadi, south of Hurghada. And I was surprised at the number of guests with knives on their BC's, or even strapped to their legs. Not many, but many more than strict enforcement would imply.
I asked one of the divemasters about that. He thought at first I was asking about him (I actually hadn't seen him with a knife); he told me that divemasters and above are authorized to carry knives, but not guests. It's supposedly not just so that tourists don't poke at the reefs; they're also afraid that a knife could slip out of a sheath, fall on a reef, and accidentally damage it that way.
But I pointed out that a couple of guests on the boat had knives, attached or strapped-on openly. He admitted that the regulation was not strictly enforced. I suppose it would be different if they actually saw a guest prodding at the reef, or cutting a piece of gorgonian...
Mania is certainly correct, that a cutting tool is unnecessary for diving at the tourist locations there...
--Marek
