Blood Beach (1981) on Netflix

Harry Caulder (David Huffman), a harbor patrolman, leaves his Venice beachfront home and greets his neighbor, Ruth (Harriet Mendin), and her dog, Piper, as he heads across the sand to swim to work. As Harry swims off, Ruth is suddenly sucked into the sand by a unseen force. Harry tells police officers Sgt. Royko (Burt Young) and Lt. Piantadosi (Otis Young) that he heard Ruth scream, but could not find her, and she would not leave without her dog. Harry knows her well because years ago he almost married Ruth's daughter, and he plans to keep searching.