We use this chip and have though about how we might waterproof them ourselves. Here is someone who does it for us. amazon 
100% Brand New and High Quality Here is a DS18B20 original chip,three wire temperature sensor High quality stainless steel tube encapsulation waterproof moistureproof prevent rust Stainless steel tube size:Approx.6*50mm(Diameter*Length) 9 ~ 12 adjustable resolution Power supply range:3.0V-5.5V Operating temperature range: -55°C - +125°C (-67°F - +257°F) Storage temperature range: -55°C to - 125°C (-67°F - +257°F) Accuracy over the range of -10°C to +85°C: ±0.5°C. No other components, unique single bus interface Output lead:red (VCC), yellow(DATA) , black(GND) Cable length:Approx.100 cm Color:As shown