Since the series was only supposed to last 3 seasons, the last shot of 3rd season was the sign outside the school saying "The End." The first shot of 4th season is the same sign saying "Not the End."
Ron asks Kim at the end of the episode whether her water skiing over a shark was a dream. This is a reference to "jumping the shark", or continuing a show after many people feel it should have ended. Because Kim denies this (by hanging up the phone) the creators are saying that Kim Possible is not over and that this is not jumping the shark.
Starting with this episode the end credits include a small scene relating to the episode.