Someone depicted as being refused that water by a man who directs him to his brother, who also refuses him and sends him back to return with something from the 1st brother - you see as stupid?
But, the 1st man who refused him the water and instead sent him on a wild goose chase - you see as clever?
Get real!
While the brunt of the deception in the story was on the Muslim, the judgment provoked by the joke is clearly against the deceiver - "the Jew."
How can some people be honestly so clueless? Or are they really?