刻舟求剑

A man from the state of Chu was crossing a river. In the boat, his sword fell into the water by his carelessness.   Immediately he made a mark on the boat. "This is where my sword fell off", he said.   When the boat stopped, he jumped into the water to look for his sword at the place where he had marked the boat.   The boat had moved but the sword had not. Is this not a very foolish way to look for a sword?