When choosing a dive knife, there are several factors you need to consider. The most important thing is to find a knife that's comfortable to hold and easy to use. The blade should be sharp enough to cut through things like seaweed and fishing line, but not so sharp that it's likely to cause injuries. It's also important to find a knife that's the right size for you. A knife that's too big or too small can be difficult to use, so be sure to try it out before you buy it.