Fear of Dark Water

Ever hear of Jenny Greenteeth? That's a kind of hag that lives in dangerous pools, so the folktales say. She grabs kids by the ankles and drags them down to drown.
Of course we know how stories like that get started. They were invented to keep kids from danger. You know, the bogeyman strategy. "Don't you play near that pool, or Jenny Greenteeth will get you!"
Only, suppose it's not a story? Suppose it's real?
Jo's fear of dark water is not just fear, it's a full-blown, stomach-churning, paralyzing phobia. Determined to beat it, she dares to swim in this forest-shadowed pool, with its ink-dark surface that never reflects the sky.
Under the surface it isn't really dark, just dim and green. The surface glimmers overhead. Jo looks down. Silver minnows fan out from beneath her feet. Below them the green light fades into darkness.
Deeper still, at the farthest edge of the light, a pale shape moves. Not a fish. Something larger. It rises toward her.