The sea symbolizes rebirth because, first, it was one of Edna's first accomplishments that set her aside from co-dependence: the fact that she would swim.
Second, the sea would always be a focal point of relaxation for her.
Third, when the end came and Edna realized that her hopes and illusions were never going to be fulfilled her way, she opted to bare herself completely and take a long swim. In the nursing waters of the ocean, she was able to realize her fate, and she figured that in order to end a life of dissatisfaction she could give herself to the waters as if in a form of baptismal redemption.
She had no fear when she realized that she was too tired to return to shore. Equally, she was too worn out in emotion to return to life as she knew it. She had awoken to her real self and it was nearly impossible to go back to a life she utterly disliked.
Hence, the ocean was Edna's way to give herself to the ocean, die embraced in the ocean that signifies freedom, and perhaps be reborn into a free spirit. Which, is what Edna really was.
Edna Pontellier has long sought to be free in the oppressive society in which she lives. Her husband, Leonce, is regarded as a good husband, but he is oblivious to her true feelings and enjoys a great deal of freedom while making his wife wholly responsible for the children. Even while vacationing at Grand Isle, Edna begins to feel depressed. "An indescribable oppression...filled her whole being with oppression. It was like a shadow, like a mist passing across her soul's summer day," Chopin writes. Edna is overtaken by dread and wants her independence, but affairs with Robert LeBrun, whom she loves and who leaves her, and another man do not bring her the happiness and freedom she desires.
At the end of the story, Edna still feels oppressed by the roles she must fulfill as a woman. As she is wandering along the beach, her children appear before her like "antagonists who had overcome her...and who sought to drag her into soul's slavery for the rest of her days." Feeling that she will never be free, she takes off her clothes and stands before the ocean. Chopin writes, "For the first time in her life she stood naked in the open air." The freedom and nakedness that Edna experiences are like a rebirth. The thoughts going through her head are of the freedom of childhood, such as the "blue-grass meadow that she had traversed as a child." As the water transports Edna away from the restrictions of the shore, it's almost as if she is going back to childhood and then to the safety of the womb. Only the water can free her and give her a kind of symbolic rebirth that life on shore, filled with male domination, cannot give her.