I completely concur with the answer above. The religious overtones and allusions throughout this story are easily missed but are essential to understanding the central idea of three "good" kids who turn "bad" only to realize the error of their ways and choose to become "good" again.
The putrid waters of the lake and the encounter with death (the corpse) are all warnings of the consequences of these "bad" actions and represent the "depths" to which the narrator descends. However, the baptismal power to cleanse him of his guilt is also there and offers him the opportunity to re-enter the world with a second change.
The lost car key, then, represents a kind of Holy Grail of salvation as well as it provides the means to finally escape and leave the lake. The key remains hidden throughout the night and then reappears to the narrator when it shines like a "jewel" in the first light of morning.
Caught trying to rape the girl in the blue car, the narrator and his friends run off into the woods, into the water. Waiting in the filthy lake, the narrator is grateful to be alive and feels horror at the death of the “bad older character” whose body he meets in the slime. His rebirth has begun. Most readers miss the "baptism" of the narrator as he trips over the body of Al. He won’t “turn up”—except perhaps in the most grisly way. It is this knowledge and the narrator’s new reverence for life that make him think he is going to cry at the end. The water of Greasy Lake has been his external "baptism" and his tears will now become his internal "baptism." The narrator has truly undergone some type of religious salvation, for he will not be spending the rest of his life in jail for murder and rape.