When a group of holidaying students wake up to find themselves buried up to their necks on an isolated beach amidst the rising tide, they are left with nothing but a note telling them they are being watched, and among them is a mole. They must figure out who that is if they want to survive.

Hello, this is a great?beginning. An open air contained thriller. ?With a thicking clock (the tide). You have the initial situation, the next step is to find the end. Who is behind all this? Why? Is is a elaborate prank ? Is it a matter of revenge? I think you need falshbacks, like in the first Saw movie.
I think the best method to build a story around this idea is to make one small creative decision at a time and continue until you have all the main elements. If you’re not satisfied start again, change something, start again, and again. It needs time. Think about what you like, what you know – are you an expert in videogames? sports? Have you been betrayed? ever think ?about a painful revenge?