Limpet mines don't work that way. They're attached by divers/saboteurs and they don't float. For sinking a ship, several are attached under the waterline. For making a statement and sowing intimidation without getting into a shooting war you can't possibly win, you might attach one a couple of feet above the waterline to cause damage without actually sinking anything.
Drop it in front and you'll have an explosive device lying in the mud on the bottom of the waterway, safely away from any and all boat hulls as the target sails blissfully away.
Thanks.
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