Siryn by C.E. Murphy

The waters have risen, leaving cities drowned and a future changed beyond recognition. And into that future, something new has been born—or perhaps something impossibly old has been released.

She does not know how she came to be, only that the oceans are her favored home...until a man from the world above nearly dies in her domain, and she is drawn toward the soft and easy life he seems to possess. But there is a price to pay for love, and in the end...

...you will never be part of her world...