There's a nod to one of the developers at Insomniac Games in Mystic Marsh. Once you start the level, jump inside the fountain which has these four coins in it. Press 'triangle' to take a closer look at these coins, and you'll see that it has the face of a man on it. This man is Dan Johnson, a guy who worked on several games made by Insomniac, including the Spyro games on the Sony PlayStation.