What causes Rachel's fear of the ocean in the novel, Words in Deep Blue?

Rachel loved the ocean more than anything— she wore swimsuits under her dresses, swam with her brother and mother, and was even planning to become an ichthyologist. She was almost one with the ocean. However, after the ocean takes Cal’s (her brother's) life, she becomes wary of it. The thing she loves has hurt her, and h.er fear increases until she is no longer able to go into the ocean. She can only to sit on the beach and mourn from afar.