We learn about the title in the fourth paragraph. The night Deanna's dad catches her and Tommy in the car, Deanna says:
[…] in my head I wrote the story of a girl who surfed the cold green ocean, when one day she started paddling in the wrong direction and didn't know it until she looked back and couldn't see the shore. (Prologue.4)
It's a metaphor for Deanna's life—she started swimming in the wrong direction when she hooked up with Tommy and can no longer see her former self. Throughout the book, Deanna refers to the girl, a (semi-) fictional character that feels the feelings Deanna represses.