He is all at sea as to what to do next．

他惘然若失，不知下一步该怎么办。

　　all at sea，原意是“在海上”，引申意为“迷茫，不知所措”。

例句：There's so much materials to be prepared that she's all at sea to fix out.
　　有太多的材料需要准备，以致她茫然无措，不知如何下手。