Kural - 929

Like him who, lamp in hand, would seek one sunk beneath the wave
Is he who strives to sober drunken man with reasonings grave

Explanation
Reasoning with a drunkard is like going under water with a torch in search of a drowned man.

Tamil Transliteration
Kaliththaanaik Kaaranam Kaattudhal Keezhneerk
Kuliththaanaith Theeththureei Atru.