人不可貌相，海水不可斗量
rén bù kě mào xiàng , hǎi shuǐ bù kě dǒu liáng
you can't judge a person by appearance, just as you can't measure the sea with a pint pot (idiom)
person (人) + appearance (貌) + sea (海) + pint pot (斗) = never judge by surfaces
The idiom pairs two parallel impossibilities: sizing up a person by looks, and measuring the ocean with a small grain scoop. Both halves use 不可 (cannot) to insist that depth — of character or of water — defeats any shallow measure.