I don't know if the A419 is waterproof, but looking at mine it doesn't seem to be as the probe tip is simply crimped to the wire lead. It appears to be stainless, but not sure about that. Why not just run a smaller blow off tube and connect it to the hole in the stopper where the airlock is supposed to attach? The smaller blow off tubes are more prone to plugging, but it's also fairly easy to check them and clean as needed for the first few days of the ferment when it's most active. Once it slows, you can replace the blow off with a dreaded air lock.