She's a Whale Watcher
At the end of the book, Dolores comes face to face (or face to blowhole) with another whale. Unlike the Wellfleet whale earlier in the book, though, which Dolores pretty much watches die as she yearns to die, too, this whale is alive. When Dolores sees it while on a whale-watching boat trip, she's the first one to spot it: "Something is happening—something private only I can feel" (29.78), she says. It's called symbolism, Dolores.
The whale comes after Dolores has finally put her life back together. While she'll never forget the pain of her mother's death, her rape, or her failed marriage with Dante, she's learned to cope with it all, and she's found support with Thayer, her new gentleman companion. The whale is graceful and free, breaking the waves and showering foam all over the place. It's little bit sexual, and a little bit full of birth imagery, but either way it represents Dolores's breakthrough, and gives us hope that she will have a nice life ahead of her after the book ends.