Many posters on these boards put too much emotion into a dive knife. To some, it marks you as a reckless attacker of women and children, to others you're a "want to be Rambo". These emotional excuses, bring about ideas of not carrying a knife anywhere, to carrying a blunt tip knife made out of soft material that won't cut.
It's a TOOL! If you are diving where there are fishermen, you need a knife to cut line, If you become entangled a knife will cut mono line, and wire cutters if they are fishing with wire leaders. Are you going to be diving around boats, or from a boat? Small boats in protected waters use thin plastic rope and small knives will cut that. If you are around bigger boats and they use larger rope,you need a larger knife.
There are lots of tools out there that won't do any job. Get a good knife that will cut the biggest rope you're likely to encounter in your dive area, keep it sharp and waxed to reduce chance of rust. Many knives have a hook that is called a line cutter. If the blade is made of soft metal, the line cutter won't cut, and the line cutter won't cut rope anyway. Many people here have mentioned EMT shears, they'll cut small line but many cheaper shears are soft metal stampings and have a weak aluminum rivet, and won't cut. If you've met emergency workers,they can show you good brands that will even cut small rope.
Each part of the country has different diving conditions, prepare for your conditions and change if you go somewhere else. . . A very small number of California divers, dive in kelp, long tough plants that can catch on a knife on the outside of their leg, they carry knives on the inside of their leg. Most divers are not diving in thick weeds, and the preferred carry is on the outside of the leg, on the calf or thigh. If you are unsure of your local conditions ask at your dive store, and put the knife where they recommend.
There is a lot of good information in these forums, just remember, you're not diving in the same places all of these people are. Wear the equipment that is appropriate for your area, not theirs.