I rigged a water level sensor in my pond using this float sensor - http://www.amazon.com/uxcell®-Horizontal-Liquid-Switch-Sensor/dp/B00AKW29U2 wired to a GoControl window/door sensor that is enclosed in a water proof box.
I created an automatic fill, using a sprinkler zone valve wired to a transform which is plugged into a GE outdoor plug module.
Edit - Just wanted to note… I don’t rely on the switch to turn off the fill. I use Smart Light Timer, X minutes unless already on - https://github.com/SmartThingsCommunity/SmartThingsPublic/blob/master/smartapps/pope/smart-light-timer-x-minutes-unless-already-on.src/smart-light-timer-x-minutes-unless-already-on.groovy
I also have it set to only run in Home mode, and also have it set to notify me when the automatic fill starts to make sure the pond doesn’t overfill 