Hm, not too much unknown. You already know what the movie's about before you watch it. Probably would be about the scariest thing in the world if it actually happened to you, watching it not so much.
Scariest part of that movie I always thought was when it ate that one guy with the kid watching and you see his leg floating down to the bottom. The leg wasn't scary, I mention it so you know which one I mean. That scene had a kind of mood where you know something bad's happening but it's too quick and there's too much going on so you can't quite get your head around it before it's over. I find that a very unsettling sensation.
Think that was the first clear view of the shark you got, as well. Sharks can be some scary bitches. Prehistoric, you know, and primal, and when you encounter them it's in their element and not yours. Probably about the scariest natural creatures under the right circumstances.