Think you can make a basic (uncalibrated) electric conductivity meter quite easilily
GND -------C B------A----[ R1 ]----- +5V
A = analog in of Arduino.
B, C = connectors to hang into the water
R1 = potmeter - should be in the same order as resistance BC with a minimum of 1K
Water between B & C will have a certain conducticity or resistance. Together with R1 it forms a voltage divider.
If BC conducts well the analog port will read a LOW value and when BC has a HIGH resistance it will read a HIGH value.
try if this suites your need.