You may recognise the first half of this proverb, as an English saying. I really like the addition that the Chinese have made to this common proverb. It really makes the meaning much more clear. There is much more to a person than you can possibly measure by their appearance.
rén bù kě mào xiāng ,hǎishuǐ bù kě dǒu liàng
You can't judge a person by appearance, just as you can't measure the sea with a pint pot.
Variation: You can't judge a book by its cover
Also see Confucius' proverb about beauty