You may find this page useful:
Currents and Tides - I Boat NY Harbor
Also note, the UN general assembly is back mid September and this may impact the east river traffic.
I did the trip many years ago in a 24' and it there were no issues as i timed the tides as recommended. It was a highlight of my trip from Boston to NJ. ( I plan to do that part trip again this weekend in both directions.)