If your sensor is in water that is deep enough so that ice doesn’t form around the sensor itself then you should be ok. If ice forms around a sensor itself that can be a problem. The Meter Group Hydros21 CTD sensors have pressure transducers that can break if ice forms around the sensor. The new model CTD seems to have a pressure transducer that is less vulnerable to freeze damage than the previous model but I think that’s it’s still not totally clear. @shicks can probably provide the best answer.