Working on your horror story or screenplay can be a frightening prospect. By horror standards, your screenplay is excellent. In his book, Steve Neal provides a detailed description of these criteria. As explained by Steve Neal in his book Genre & Hollywood, horror films must follow a four-layer plot; starting with "start", followed by "discovery", "confirmation", and "encounter". You must have the characters in your story discover something new. Afterwards, they acknowledge it exists and try to cope with it. While the rules are correct, I think the dialogues need to be a bit changed to fit the nature of the characters. Build your story out, accentuate the characters, find the crucial moments where we learn something. And let the idea take you where you want to go. Creating the atmosphere was very successful, and you were able to borrow ideas from Steven Spielberg's films like "Jaws" and "Ghost Ship". Wishing you success!