Water conducts electricity. In fact, it has to if you are not supposed to pour water on your computer :lol:
So, the absolute, most simplest way to do it is to use any non-metallic container (a container which cannot conduct electricity).
In your circuit, you will have two long leads. Stick one at the bottom of the tank. Make sure the bare wire can reach the water.
Now, stick the other lead (or wire) at the highest point you want the water to go before the motor cuts off.
Now make an inverter with transistors (you can find it on play-hookey.com) and connect the battery, the inverter, the motor, and the water in series.
You might need to somehow give extra power to the motor, because it doesn't like to run on low power.