Sorry it took so long to get back to you on this - I wanted to peep into the files and give you an accurate answer before I said anything.
Right next to the waterfall, there’s a thing sticking out of the ground. Check that out and you should get the key you need! (If you haven’t gotten through this part already 
) The rusty key is a key to something else entirely, but it was meant as a diversion for this room specifically.
Sorry again for the delay!