I found it interesting that the basic plot twist of the murder predates Chinatown by two years.
In TOKTM, the coroner discovers that the body found in the ocean drowned from fresh water.
In Chinatown the coroner tells Jake Gittes murder victim Mulwray's lungs were filled with salt water, despite his being found in a freshwater reservoir.
Coincidence? I wonder...
The underlying plot in Chinatown was water though. The 'founding fathers' of L.A. were thieves stealing water from the rest of the state. Having the murder plot twist tied to it only emphasized the importance of water in the plot. I really doubt the writers of Chinatown thought to copy TOKTM, do you?
shareI can't say for certainty, but it seems to me that crime movies much earlier than either Chinatown or They Only Kill Their Masters have used a similar plot device, where a coroner discovered salt water in a dead person's (or drowning victim's) lungs when fresh water was expected, or vice versa. So, I don't think either movie had an original plot device there.