Water overflow pail detector
LEVEL DETECTOR TO SOUND AN ALARM WHEN A SLOWLY FILLING PAIL OF ATE RIS ALMOST FULL


Google: water level detector circuit
Click on "images"


MANY water level detector circuits each with its own web page.

This should suit your need.





A slightly different way:

N1 can also be 74C14, 74HC14 or any "Schmitt trigger"  CMOS gates. 





This circuit and several others here





Float switches:

Have a look at these images  "float switch" .
 Each one of these images is hot linked to webpage. 

These are probably even more likely to be apposite "float switch" bucket

From the above link:









Using "float switch" low voltage 

This looks about what you want. Discussion here



All these come, as you will have seen if you looked, by using a simple Google search with some obvious terms and then looking at "images". A very powerful tool. Well worth spending the time required to learn how to use it.