Jenny was only underwater for about 3 seconds when Jeb grabbed her and pulled her to safety, yet she was still unconscious. It's almost impossible for her to have lost consciousness after such a short time underwater.
Reply: The human mind is a pretty powerful thing. Psychologically, if a little girl was making a suicide attempt and meeting her own death like that, it's very possible that she tuned herself completely out from the rest of the world--like an out-of-body experience, almost. That could be why she also didn't respond to Jeb screaming her name and running into the water after her. I'm not a psychologist, and I don't know if this is true, but I think it's a possibility.