There are different points that you should pay attention to while choosing the right diving knife that will serve your needs. I would like to list a few tips that will help you with your choice.
The first parameter that matters is the size. Many people believe that the larger the better, yet this is one of the most common misconcept that could lead to many inconveniences. You will feel very awkward if you put too long a knife on your BC. It can cut you or damage your equipment. Remember that you are not an underwater warrior, and do not have to fight your imaginable opponents. Large knives are those that have a blade more than five inches long.
Medium-sized knife (with blade of four to five inches) is practical in most cases. It is easy to put this knife on your BC, arm or leg, and is good for a firm grip, large enough for a grip. Be sure to check the length of a handle, it should be comfortable for your hand and large enough.
Mini knives have a blade of two to three inches, and even can be clamped onto a hose. Check the length of a handle, it should be no less that three inches in length, otherwise your hands or gloves can be damaged by blade.
With shape of the edge there are basically two options, straight or serrated edges. You select a diving knife with a straight edge for cutting plastic, nylon rope or monofilament fishing line. Yet serrated edge is more efficient when you cut natural fiber rope or kelp. Look at serrations that should be located along the whole length of the blade. It enables a good sawing motion for great results.
Many knives have both straight and serrated edges making it a universally adoptable tool, efficient in different situations.
There are different types of tips as well. You can see either a blunt tip or a sharp tip. For spearfishing tasks you select a knife with a sharp tip. For a whole bunch of other tasks such as digging or prying, chiseling or hacking you would rather use a blunt tip. There are different problems with a sharp tip. For example, it can be easily broken or can cut you or puncture your BC. At the same time a blunt tip is both safe and efficient as a tool.
You should consider all of the above to select the right diving knife serving all of your purposes and tasks.