This is a simple sensor you can print out to notify you when the water level in a container drops below the desired threshold.
You will also need three metal washers (1/4" inner diameter) and some wire.
Print out the float-allparts.stl OR three washer holders and one of everything else. Do not print float-assembly.stl as that is just for the pretty render and to guide you in putting this thing together. Assemble as depicted in the photo and/or float-assembly.stl.
Cut a small hole in the lid of your container and snap the center and center bottom together so they hug the hole. Cut some 3mm filament to the desired length-- this determines the water level threshold. When the water level drops below this threshold, the float will pull the top washer down onto the other two completing the circuit. I hooked the positive end of the circuit up to the top washer and the negative/ground to both of the lower washers (rather than hooking up the bottom washers to one each) since curved filament or an un-level container will cause the top washer to rest on only one or the other bottom washer. Using a second (shorter) sensor as a control can prevent false positives and/or provide an upper threshold if you are using this in conjunction with a flow control valve like http://www.thingiverse.com/thing:8394.