The Man From Atlantis (1977)

A great tidal disturbance deep in the Pacific Ocean unearths the sole survivor of the fabled lost kingdom of Atlantis and washes him ashore. When the Atlantian is brought to the Oceanic Research Foundation in southern California and falls ill, Elizabeth Merrill, a doctor who names him Mark Harris, saves his life by returning him to the sea. Mark decides to remain with the foundation to help further humanity's knowledge of the sea.