I have the same questions/comments as yqwertz and variable plus a few more.

Someone in the group needs to be the protagonist. They need a leader and the logline should focus on this person. Who’s going to lead the group? Or, as it’s a horror, who’s the last person standing? The leader is a good person to kill in a horror as it ups the stakes.

I’ve gotta ask – how can a group of students afford to throw a party on a luxury cruise liner….? A cruise liner suggests a ship that takes hundreds of people around the Caribbean (or similar). A luxury yacht would make more sense to me. If it’s someone’s Daddy’s yacht immediately I have nice mental image of the group of people we’re talking about too. This could just be my interpretation but my google image results show lots of pretty huge ships…

The inciting incident needs to be something specific. Something that can happen in one scene. Like “When the body of a student is found with a message cut into his flesh…” Think visually – what’s that one moment that kick starts the plot? As they’re on a boat and everyone’s gonna be wondering why they don’t just turn the boat around and head home maybe the stowaway sabotages the boat and the radio and they’re stranded at sea. Perfect! Monster in the House!!

“Event slowly turns into a nightmare” – Don’t use slowly! It’s a boring word that makes me think that I’ll spend the first 60mins waiting for something exciting to happen. What I really want is a quick, sharp shock that flips the story in the first 20mins or so. It’s a perfectly normal party then BOOM it’s a nightmare. If you’re planning on teasing the group, making them paranoid and drawing out the reveal then that could work BUT I wouldn’t describe this as happening “slowly” as there will be still be a turning point – that quick, simple moment when they realise they’re not alone.

How is this going to be different to other teen slasher films (scream, I know what you did last summer, etc)? What’s the hook?

Overall I think you just need to focus more on the specifics – wreak havoc how? punishes the students how? Sounds cool though.