Vita Nostra by Marina & Sergey Dyachenko
A sixteen-year-old girl’s life takes some very weird turns when she notices a strange man watching her during her beach vacation with her mother. Every time Shasha successfully evades the stranger, she awakens in a terrifying time loop until she finally agrees to hear his bizarre request.
At precisely four in the morning, Shasha is instructed to swim out to a buoy in the ocean. When she complies, she receives three gold coins and is ordered to continue the practice daily. But when she oversleeps one morning, a close family friend pays the consequences.