Old ending explained · Cinespective

What happens

Trent and Maddox swim out through the reef and surface as middle-aged adults. They learn the resort has been testing drugs on guests, using the beach to speed up trials. Investigators arrest the staff, and the siblings go home with the truth.

The last image

The two siblings, now middle-aged, stand together in daylight, safe at last and left with everything they lost on the beach.

What it says

The ending turns a nightmare about time into one about exploitation: the beach was never fate, it was a business. The survivors escape, but only after losing their parents and their lifetimes, so the cost of their rescue stays with you.