@marylu 我也有同样的问题,所以就顺手谷歌了下,http://idioms.thefreedictionary.com/ 这里提到: blow someone or something out of the water
Fig. to destroy utterly someone or something, such as a plan. (Alludes to a torpedo or other weapon striking a ship and causing a great explosion that makes pieces of the ship fly out of the water.)
大意是这句短语来源于海上战争,描述一艘船被鱼雷或其他武器击中时发生大爆炸使得许多船身碎片飞出水面的情形。想想也是蛮贴切的,这句短语主要就是表达to destroy or defeat something or someone completely的意思。:)