When looking at the poster for the film on June 13, 1931, Marty McFly noted that it looked like they used a real shark.
Behind the scenes
Shark is a reference to the movie Jaws 19 in 2015. Marty mentioned that the shark in that film still looked fake, despite being scared when the holographic shark came towards him. In contrast, Marty mentions that the shark on the poster in Shark looks real.