Blood Beach (movie, 1980)

Something or someone is attacking people one by one on the beach. Some of them are mutilated, but most of them are sucked into the sand, disappearing without a trace. What is the creature responsible? Where does it live, and where did it come from? And is there any chance of it reproducing? Meanwhile, David Huffman and Mariana Hill are once-almost-married old friends, reunited over the death of her mother on the beach, and searching for clues in the abandoned buildings where they used to play when they were young.