In retrospect, if Noah had just chucked this dead dude's corpse into the ocean, Jake Gittes never would've been able to figure out most of the mystery. (Hindsight's always 20/20.)
After the police pull Hollis' drowned corpse out of the reservoir, Jake goes to see the dead body at the city morgue. The mortician—appropriately named Morty—notes the irony of the situation, commenting to Jake:
MORTY: Can you believe it? We're in the middle of a drought, and the water commissioner drowns. Only in L.A.
The method of death is something that seems logical on its surface, but the cops are able to look into it enough to realize that someone set Hollis up for adultery to make his death look like a suicide. Yet, they don't discover as much as Jake, and don't realize that Evelyn didn't kill Hollis—the murderer was Noah.