Dark Heart, a super-powerful Eldritch Abomination, hits his head on a canoe and nearly drowns. How does that work? Two theories come to mind: one, while he's in a certain form, he has that form's weaknesses. Two, maybe his developing friendship with Christy was starting to affect him, and he was already beginning to become a normal boy.
His hiccups, inexplicable sleepiness and tendency to shapeshift into random forms near the end of the movie also has something to do with caring getting to him: he's acquiring human frailties.
Just as Dark Heart's lair was held together by his evil, Christy was being poisoned by Dark Heart's evil powers of not-caring - she didn't care enough about surviving. Dark Heart's powers were fuelled by evil, but he gave up his vast power because he cared so much. Once his powers were gone, his evil was no longer poisoning Christy, and the caring powers of the Care Bears were able to work. We didn't need to cheer Christy on; we needed to encourage Dark Heart!
Christy not wanting to leave Dark Heart's side and lose her newfound abilities gains an extra layer when you take into account that she was mocked and bullied by the other kids for being a loser prior to joining his side. Since Christy's been a jerk to everybody as Camp Champ when getting her new abilities, if she loses her new abilities it won't just be that people will think she's a loser; they'll know that she's also a jerk and will likely treat her accordingly. No wonder she doesn't want to go back to being normal.
