Secrets at Mystic Lake by Tiffany White
This was supposed to be their last summer together. Five best friends have graduated college and are about to go their separate ways. For one last memory together, they go on a camping trip deep into the mountains, far from civilization.
There they discover a mysterious lake, described only in legend, with the supposed power to transform any who enter its waters into the opposite sex.
As a joke, Joe throws Sam in to find out. Within seconds Sam begins changing, and right before everyone's eyes, turns into a beautiful woman. Joe finds it absolutely hysterical -- until Rose pushes him in too! And to Sam and Joe's surprise, the change includes a heightened sex drive, that they both feel an immediate need to satisfy.