Night Tide

Night Tide is a 1961 thriller film, written and directed by Curtis Harrington and starring Dennis Hopper. It was filmed in 1960, premiered in 1961, but was held up from general release until 1963. The film was restored by the Academy Film Archive in 2007.

Mora believes she is a descendant of the Sirens, mythic sea creatures who lure sailors to their deaths. From time to time a mysterious and seemingly sinister woman Marjorie Cameron appears and frightens Mora. She believes the woman is the leader of the mermaids, calling Mora to fulfill her destiny. Moras origins seem to be driving her to commit murder by the full moon. Johnny is unable to believe his lover is capable of murder, but Mora herself seems more certain. During a diving trip on the day of the full moon, Mora cuts Johnnys breathing tube and he is forced to surface, leaving her below, where she swims away into the shadows to die herself in order to save Johnny from future harm.When Johnny returns to the marina the next night, he sees that the lifeless body of Mora is now on display in the mermaid tank. Murdock appears, brandishing a gun. It was he who committed the murders and convinced Mora that she was an actual mermaid. Murdock himself was in love with Mora and his jealousy drove him to kill the men Mora was interested in. Mora had discarded her oxygen tank in the sea, and had drowned. In the ensuing struggle between Johnny and Murdock, the glass tank is shattered, and the water and debris pin Murdock down. ........

Source: Wikipedia