u/Horror_Shine_6398

Just wondering since I’ve finished act 1

I distinctly remember this film I saw as a kid on tv about a dad and his daughter moving to a new house near the ocean. The daughter gets lost, and he later finds her and she’s become either a monster or indoctrinated by a monster. I remember it being terrifying when he goes underground and finds her the way she is. I distinctly remember the dad being a single parent with just a daughter. Anybody know this one?