In the Coyote's first attempt to catch the Road-Runner, he is shown standing on a bare patch of pavement surrounded by the puddle of glue he is spreading. When the scene cuts to a long shot for the "beep-beep" sound, it returns to a close-up of the Coyote standing on glue with no bare patch. There is another cut to show that the approaching object is a truck and not the Road Runner, and when we next see the Coyote, he is standing on bare pavement in the middle of the glue puddle once again.
The outboard-motor-in-the-water-tub method would not work, because the thrust of the motor against the water would drive the water against the end of the tub - thus forcing the motor forward while driving the tub backward. Those two forces would cancel each other out.