Birdy and the Ghosties by Jill Paton Walsh
She looked once, and she saw the terrible ghosties...Sometimes Birdy watches her father row people across the dangerous sea, but when the wrinkled old woman asks to be ferried across, Birdy jumps in too. The woman tells Birdy she has second sight, but Birdy isn't sure she wants this special gift. However, soon she finds that looking twice at things can bring the most unexpected results...