What is someting in the Greasy Lake story that many people miss the meaning of?

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.