The idiom is "刻舟求剑" (Kè zhōu qiú jiàn), which character-by-character literally means "carve-boat-search-sword", or as the iPhone Pleco app puts it: 

"nick the boat to seek the sword" (make a notch on the side of a moving boat to locate where a sword dropped overboard)-- an action made pointless by changed circumstances. 

In Yu Hua's New York Times editorial on censorship in China, he likens the Party's invitation of criticism from the people and the subsequent censorship of public blog posts to this idiom; the result was that no real criticisms could be found in public forums.

If you don't have the free Pleco app yet, I highly recommend it as it handles idioms quite well, even if you only know the pinyin (as in this case)!