A simple, versatile circuit which indicates the level of water in a tank is detailed in the link given above. This circuit produces an alarm when the water level is below the lowest set level and also when water just touches the highest set level. The circuit is designed to display 12 different levels. However, these display levels can be increased or decreased depending upon the level resolution required. This can be done by increasing or decreasing the number of level detector metal strips and their associated components. A tone generating IC is used to sound the alarm.
Another link with a circuit diagram and explanation is available at