Waterrat (2018)

Cindy and her younger sister Rosa go swimming in a small, remote pool. By mistake, the desk clerk thinks the two are long gone, while they're actually practicing a breathing exercise under water, and shuts down the building. A military experiment opens a dimensional door and releases an alien entity that attacks the girls and appears to be invisible above the water surface. With their mobile phones stowed away in a locker, the locker key lost on the bottom of the pool and Rosa helplessly floating around on an air mattress, Cindy must find a way to safe her sister and get help from the outside.