Lighting is the most effectively used technique in the first scene of Jaws. At the start of the film we see a group of teenagers around a campfire in the night. This tells us that
1. They only light there is the fire and the moon
2. This restricts the audiences and the characters eyesight making them appear more vulnerable.
These two points work in conjunction together and build suspense.
When Chrissie enters the water she is naked and therefore, sets up the attack as man versus beast and she is completely vulnerable. To make it her seem even more helpless the boy passes out on the beach. She has no help what so ever. The camera shows her to be playing around in the water and having a bit of fun so Spielberg makes the audience feel more relaxed. Suddently the camera changes into and undershot showing Chrissie's naked legs and makes the audience become more anxious that something is in the water with Chrissie. The music also builds and then shes is attacked. I read something quite interesting on this and instead of Chrissie being attacked she is raped. This is juxtaposed with the orgasmic sounds - "Oh, God" , her screams and the fact she is naked.
Anyway back to the question, the fact that Spielberg doesn't show any part of the shark, does not confirm that it was a shark. This may sound stupid because of the cover of the Dvd packet and the title.