Nice logline!

I think the story really starts when Gittes finds out that Evelyn Mulwray (is that her name?) is not who he thought she was. So this is the inciting incident.

Try something along the lines of "When a [what is his flaw?] private investigator learns the real identity of an important client, a case of adultery turns into a murder,?and he finds himself up against the most powerful Los Angelinos?in a?widespread water scandal."

It's a clunky start, but I think it has a few elements that are essential to this movie.

Perhaps,

A private investigator hired to expose a philanderer, becomes embroiled in a murder and a web of sinister events, culminating in an unforeseen tragedy.