This led to having Susan hooked up with cables to have an off-camera crew drag her around, and a camera right at water level to put the audience treading water right with her. But the scene succeeds or fails not with special effects, but with "Crissy" selling it to the audience that she was being eaten by a shark.
The audience is hooked at this point, and is at the edge of their seat from then on.
It is a *masterful*, terrifying scene.
One thing Speilberg did that was so effective in that first attack scene was that he moved the victim suddenly, laterally through the water at the speed that an attacking shark would swim. This projected the power and size of the Beast (as well as sharply limning the helplessness of the girl) without showing the animal at all.