What does the sea represent for Franny?

Franny loves the sea "with every breath of me, every beat of me." What does the sea represent for her? 

The sea was a source of power that she respected and where she felt most at ease, with its tides, and movement and ability to take away life but it was not something she feared. The sea was life for her and being near it was critical for her. It also represents the dangerous state of the earth that needed to be respected or it could take life away as it was doing in this near future story.

I agree with Alison that the sea was life for Franny. More than anything she depended on the sea to give her answers and to support her needs.

I saw the sea as Franny's native habitat, if not her home. She learned how to “best it” and be “aware of its capriciousness” so that she could retreat to it when she needed to heal and recharge as most animals do.

Sanctuary. I agree with andrea. It was where she went when she needed to heal and gain new direction.

It was the last wild place - the place with the most area and most creatures still left unmarred by what humans had done to the planet. Somewhere wild creatures were still doing what they'd done for eons.

For someone who's never felt a sense of 'home' I think the constancy and reliability of the sea was what drew Franny in.

Franny finds her "home" in the sea. It is wild as she is and she feels comfortable there in a way that she doesn't anywhere else. She is referred to as a "selkie" several times in the novel. She is like those mythical creatures who shape-shift between human and seal. She has a need to punish herself, to do things that push her to the edge and the sea provides her a way to do that. The sea is also a place where she is able to do good things and save those she loves. For example, early in the novel, she dives into the sea to "save" Ennis who she doesn't even know at that point. Later she saves the crew of the Saghani by swimming to another boat and taking their fresh water back to them. Towards the end of the book, she saves Niall from drowning in the frozen lake by rushing in after him.