You should always have a cutting device on a dive. It's not for defense, it is to cut away entanglements. You never know when you may get caught in a fishing line, or even a dive flag line. I have seen it several times. What if you are on a vessel that is anchored and it starts taking on water because the line is pulling the boat down (don't laugh I know of an instance where this happened and the boat sank because no one on the boat had easy access to a knife). A cutting tool goes right a long with an SMB, whistle, and torch. You should never go on a dive without any of those, even in the middle of the day.