The Beach House, a time travel mystery romance by Bruce Allsman

A young man, a yet-to-be-famous American novelist from the 1920s, struggles to find a way out of a time travel loop, unable to get back to his own time. Despite repeated attempts through an underwater cave portal, he keeps returning to the beach house, his family's summer home, at different future times. And when he meets a young woman in the year 2010 who lives at the beach house, she tries to help him when she realizes who he really is and would become. Soon they fall in love, and besides helping him to return to his time, she receives unwittingly something special from him, a gift whose value is far greater than whatever he could imagine.