The Forest of Hands and Teeth, in Mary's mind, is the opposite of her precious ocean. In her mind, the Forest of Hands and Teeth is her village, the paths, even her family. Why? Because each one of them is ruled by the zombies. The village's rigid rules and regulations are in place to keep the human race from disintegrating at the hands (okay, more like the teeth) of zombies; the gang's time on the paths is filled with fear as they try to secure food and shelter for themselves without turning into zombie chow; and every member of Mary's family eventually gets zombified. The Forest of Hands and Teeth has slowly eaten away at Mary's life, yanking away all that's precious to her.
Mary thinks she wants to find the ocean. In reality, she wants to leave the Forest, which we think makes it important enough to earn title status.