In the final chapter, Violet goes swimming by herself in the Blue Hole. This is where she went swimming with Finch during one of their wanderings, and also the place where he took his own life. In going there, she's confronting some of her best and worst memories.
The final lines are a reference to The Waves, a Virginia Woolf book that she and Finch were always quoting to one another:
I think of my own epitaph, still to be written, and all the places I'll wander. No longer rooted, but gold, flowing. I feel a thousand capacities spring up in me. (59.6)
Swimming in the Blue Hole doesn't bring Violet a sense of closure, exactly, but it brings her a sense of peace. She accepts the good and the bad, finding a sort of balance the two. She "tread[s] water" (59.6)—she doesn't sink or float. She lets herself feel excited about the future, which is wide open. Then, out of nowhere, a shark rises out of the water and gobbles her up.
Okay, we made the shark part up.