According to this NYT article[1] the rat was indeed submerged in the movie (the scene wasn't faked). The article also states that it's been tested successfully

According to this NYT article[1] the rat was indeed submerged in the movie (the scene wasn't faked). The article also states that it's been tested successfully on numerous animals - and even one human (although apparently they almost died in the process). [1] http://www.nytimes.com/1989/08/06/movies/film-the-abyss-a-foray-into-deep-waters.html http://www.nytimes.com/1989/08/06/movies/film-the-abyss-a-fo...