Hmmm, I was looking at images of a different version of the sensor board. There is an LED on the one posted in reply #10, but not on the one I looked at most closely (similar to the "best data sheet" board linked in the OP).
I agree with the general circuit tracing for the image in reply #10, except for the missing LED and the stated values for the resistors.
The markings on the resistors can be read on the image posted in reply #10, and they are clearly 101, 101 and 102, which correspond to 100, 100 and 1000 Ohms, with the 1K Ohm resistor being a current limit resistor for the LED (board powered on).
If that is the board you have, then yes, as water allows current to flow to the base, the signal voltage should rise due to increased emitter current.