The Hardy Boys / Nancy Drew Mysteries (1977)

S2E20 Death Surf

While windsurfing in Hawaii, Frank attempts to save a girl named Maryann (Maren Jensen), who suddenly vanishes and is presumed dead. This incident ignites a deep obsession in Frank as he seeks to uncover the truth behind her disappearance. However, he's being monitored by someone else who is also searching for her, adding an unsettling layer to his quest.