My husband was water-skiing when he fell into the river. As the boat circled to pick him up, he noticed a hunter sitting in a duck boat in the reeds. My husband put his hands in the air and joked, "Don't shoot!"
The hunter responded, "Don't quack."
This joke is from a Reader's Digest. The humor lies in the response of the hunter. Though the hunter responds "Don't quack", he insinuates "I will not shoot you because you do not seem duck-like enough to me to shoot". If the person in the water were to quack, then he would seem duck-like enough to shoot, and the hunter would shoot him.
The insinuation that the hunter will shoot the person if the person quacks is unexpected, and therefore funny.