As great as the monologue is in the script, it's Robert Shaw's rewrite of it and absolutely KILLER delivery that elevates it from just another monologue to one of the greatest of all time (in any medium). I still get chills watching it...

I can't help but feel that unless you're an established writer -- a chunk of unbroken dialog that long would never make it past a reader or notes.

YES. THIS. This is one of the greatest monologues in film, the marriage of a really great script, an actor who understands and gives completely what the film demands, and a director whose ego allows the organic process of creation to unfold without stepping on it. This bit that Shaw added: "...Sometimes that shark he looks right into ya. Right into your eyes. And, you know, the thing about a shark... he's got lifeless eyes. Black eyes. Like a doll's eyes. When he comes at ya, doesn't seem to be living... until he bites ya, and those black eyes roll over white and then... ah then you hear that terrible high-pitched screamin'. ..." This is the salt on a prime cut of meat, the thing that ensures this monologue in its entirety is memorable even 38 years later. Stellar lesson to writers that sometimes our work develops a life of its own and we need to get out of its way at the moment it attains artistic self-awareness.