Just some feedback that the SmartThings Water Leak Sensor works perfectly as a temperature sensor in my freezers. Had them in for a couple of months and they all work fine at 0 degrees F. The battery level reduces to around 50% and stays there so no concerns with the CR2 lithium battery. I have set up an automation to alarm if the temperature goes above 20 degrees so a bit of peace of mind.
Note this does not work with the ST Multipurpose Sensor or Button as the button batteries will not hold charge at low temperatures.