Cathrall, Sylvie. Sunken archive #1

"A beautiful discovery outside the window of her underwater home prompts the reclusive E. to begin a correspondence with renowned scholar Henerey Clel. The letters they share are filled with passion, at first for their mutual interests, and then, inevitably, for each other. Together, they uncover a mystery from the unknown depths, destined to transform the underwater world they both equally fear and love. But by no mere coincidence, a seaquake destroys E.'s home, and she and Henerey vanish. A year later, E.'s sister Sophy, and Henerey's brother Vyerin, are left to solve the mystery, piecing together the letters, sketches and field notes left behind--and learn what their siblings' disappearance might mean for life as they know it."--Amazon