rén bù kě mào xiàng, hǎi shuǐ bù kě dǒu liáng
人 不 可 貌 相， 海 水 不 可 斗 量

 
As a man cannot be known by his looks, neither can the sea be fathomed by a gourd.

解释 ( paraphrase )
海水是不可以用斗去量的。比喻不可根据某人的现状就低估他的未来。

This proverb notes that judging by appearance may lead to serious makes.

貌：相貌
      Appearance.

相：动词，看外表来判断优劣，相貌，可以颠倒词序来理解，即看相貌，看外表。
      (here a verb) to judge by appearance; 相貌 can be understood by transposing the order of the two words.

斗： 是过去用来量粮食的器具。
       A gourd dipper used to measure grains in olden times.

造句 ( example )
人不可貌相，海水不可斗量，不能只看外表评价一个人。