MATH PUZZLE: Can you replace...

An avid duck hunter was in the market for a new bird dog. His search ended when he found a dog that could actually walk on water to retrieve a duck. Shocked by his find, he was sure none of his friends would ever believe him.
He decided to try to break the news to a friend of his, the eternal pessimist who refused to be impressed with anything. This, surely, would impress him. He invited him to hunt with him and his new dog.
As they waited by the shore, a flock of ducks flew by. They fired, and a duck fell. The dog responded and jumped into the water. The dog, however, did not sink but instead walked across the water to retrieve the bird, never getting more than his paws wet. This continued all day long; each time a duck fell, the dog walked across the surface of the water to retrieve it.
The pessimist watched carefully, saw everything, but did not say a single word.
On the drive home the hunter asked his friend, "Did you notice anything unusual about my new dog?"
"I sure did," responded the pessimist. "He can't swim."