If you want to measure resistivity of the water, and you have connected the sensor to pin and ground,
then you need a pull up resistor (between pin and 3.3volt).
The pin now should read max A/D value when not submerged, and a lower value when there is water between the contacts.
Don't convert to volts. It's not a volmeter you're making. Just work with the A/D values.
Experiment with the pull up resistor value, to get max differential readings. 10k is a good start.
You might want to connect the pull up resistor not to VCC (3.3volt), but to an output pin that you set HIGH just before reading the sensor.
That will minimise corrosion. Set the pin LOW again after the reading.
Leo..