Controlling dc motor in arduino with and without a load
As Brian mentioned, a simple solution would be to measure the motor current. Alternatively, assuming that you have a free pin to measure voltage (as this is what you mentioned), you could use a pressure transducer, which would give you exactly how much water is in the bucket. 

Arduino have a nice explanation for such sensors: http://playground.arduino.cc/Main/Waterlevel

Edit: additional information added.
A pressure transducer outputs a voltage that is proportional to the pressure. Higher the pressure, higher the output voltage and vice versa. But why measure pressure? The more water in the bucket (in height) the higher the pressure at the bottom of the tank, I suggest you look at "hydrostatics" on Wikipedia. Therefore, if you place the pressure transducer at the bottom of the bucket, the output voltage will be proportional to the height of the water inside the bucket.