What were the circumstances that led to Helen's panic attack?

Helen suffers a panic upon leaping into the sea at Cape Cod, an expedition she describes in Chapter 10 of her memoir. Helen had read a description of the ocean in a book and had developed a longing to feel it, so she was delighted to be able to touch the sea. However, as soon as she was in her bathing suit, she leaped without hesitation into the "cool water" and was taken aback by what she experienced. At first, the feeling was joyful, but then her "foot struck against a rock" and the water closed over her head. Struggling, Helen found herself unable to grasp at anything for support, with the waves jostling her from side to side and her feet unable to touch the earth. Naturally, Helen was at this juncture filled with fear and felt that she was helpless against the power of the waves. It was only when the sea threw her onto the beach and into "my teacher's arms" that she was able to recover from this sudden panic. Not having ever been in the sea before, Helen was not prepared for its power.