In the Shallows by Tanya Byrne

From the acclaimed author of Afterlove comes a beautiful sapphic second-chance love story drenched with longing, loss and mystery.
Mara's ex, Nico, is the girl of her dreams: beautiful, wild and unpredictable. She's Mara's everything, even though Mara's never sure that she's Nico's anything. Then Nico goes missing ...
New Year's Day: A girl is rescued from the sea. She knows she is called Nico, but other than that, she has no memory of why she was in the sea or what came before.
When destiny reunites them, is this Mara and Nico's second chance? Can their relationship make it out of the shallows? And what will happen when they discover the truth behind Nico's accident? Because one day, Nico will remember everything.