Have to wholeheartedly agree there would be many, many, many situations it wouldn't be effective, totally useless, especially for those a situation when there isn't even a shark around. However (yes there is always a however), there would be situations that it would be effective. Kind of like diving with a pony bottle, or always having a whistle with you, or a line cutter. Pretty much 100% uneffective since they aren't used and should just be left on the boat or at home.