Chinatown basically offers a first person limited perspective narrative—we know everything that Jake knows, but nothing more.
When he discovers that water is being stolen, we discover it at the same time, and when Jake learns that awful incestuous truth about what Noah did to his daughter, we learn it too.
Part of the suspense of the movie comes from us uncovering the mystery at the same pace as Jake. We encounter the same surprises and stumble on the same moments of insight as the plot progresses. Roman Polanski actually said that he was imitating the detective novels of Raymond Chandler in restricting the movie to Jake's perspective. (Source)