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.