Why the happy ending?
Spoilers
The comic ends with everyone dead and nothing explained. The movie ends with the beach's properties remaining unexplained but the resort using it for field testing medicine results. Also, there's an underwater escape route now. The teenagers escape and notify the police when the resort brings new guests to the beach.
The reason is of course that the US audience wants a somewhat happy ending. In Europe, the original ending would have sufficed, as the story's message is, of course, CARPE DIEM!