If there is absolutely nothing else you take away from the blog of a person who reads scripts for money, take this:
Move forward and bite.
Don’t stop to explain how the ocean works. Or how eating fish to survive works. We get all that.
When you rewrite, move forward. And bite. There will be slow moments to bond characters and reveal backstory. They are necessary, but they don’t belong at the beginning.
There is much more you know about your story than anyone will care about in the first twenty pages. In those pages, focus on action. Give us a character we don’t want to see bitten, and show the shark.
Show the situation moving forward, and biting. Show how the protagonist evades and resolves to attack. Sell your bite, not the ocean, not the shark, not the fish, sell the BITE.