There are two episodes in particular that relate to this question. The first depicts Billy's experience being thrown into a pool. His father tells him that he is going to teach Billy to swim by throwing Billy into the pool. Billy is terrified by the idea and before he realizes what is happening he realizes he is at the bottom of the pool.
Billy does not try to swim, but actually feels relieved and comfortable as he begins to drown. Someone pulls him out of the water.
The next episode comes when Billy and his parents visit the Grand Canyon. The canyon vaguely resembles a pool. Standing on the edge fo the Grand Canyon, Billy recalls what happened at the pool and so pees his pants, afraid his father will throw him into the gaping canyon below.