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.