If you have absolutely no idea what's happened to Celia at the end of the book, you're in good company. Here's Cristina García on the final scene:
I deliberately wanted [Celia's fate] to be ambiguous. I've been asked whether Celia commits suicide, or if she swims back to Cuba or even if she might get picked up by the Coast Guard. My answer is always: I really don't know. (253-54)
There you have it. However, we don't want this to discourage you from making your best arguments for your pet theories. It's clear that García connects Celia's most intimate identity with those pearl earrings, so it is absolutely significant that she releases them to the ocean—and that their shimmer is extinguished by the darkness of the waters. Make of that what you will.