The female 9 way d-type, which I understand is part of a USB-RS232 converter is NOT compatible with your arduino. Stop connecting it.
The output is +10 to -10V, which might blow the Arduino input pins.
Try this:
COM to Arduino ground (0V).
NO to an Arduino digital input pin. Any will do (but not the dedicated serial 0 or 1).
Set the Arduino input pin to INPUT and write the value to HIGH. This will turn on the internal pull up. So the value read
will be 1 when the bucket is not tipped, and 0 when the bucket tips.