a) It's going to depend on your water.
b) You don't apply a voltage directly, what changes when you tilt the bottle is the resistance between the two pins.
You should connect one pin to 5V through a resistor and the other to GND. Measure the voltage where the resistor is joined to the pin.
What value resistor? Use a multimeter to measure the resistance between the pins when the bottle is tilted, multiply by 10.