I'm judging because you can't give a simple plot. Look at some good and bad storygames on this site. Authors of bad stories are generally unable to give a decent plot of the story. This is the first sign of a bad story and I really wish some were left over from the Purge to show a good example of a bad story. Authors of good stories can actually give you a good plot that will give you an idea of what the story will have. Your two sentence plot summary will just make readers go, "Is that it?".
If the only thing you can say about your own story is, "There is a shark attack and you have to survive." then you should know that giving any decent advice is impossible. When you make your plot, you should set the scene, give us a visual, and an idea of what will happen.
Here is an example that I'm taking from The Price of Freedom: Innocence Lost
You are 10 years old when you first feel the weight of shackles around your wrists. In a matter of hours, your whole world is turned upside down as you are dragged away from your home in Greece to work as a slave in the Roman Empire.
Now you must struggle for survival. Every decision you make will affect your future. Will you make friends or enemies? Will you submit to your masters or fight your every command? Will you sell your soul to earn your freedom, or sacrifice everything for the people you love? Will you even survive long enough to see the sun rise?
This states what the story will be about, sets the scene, and gives me a visual of the story. You don't have to go super into detail, but give enough information that satisfies those three points I made.