Spa thermometers monitor the temperature of the water in order to maintain a comfortable level for you and your guests. Spa thermometers are available in chrome or plastic models. Some spa thermometers are designed to read both Fahrenheit and Celsius; whereas others read only one. You can find spa thermometers in simple designs such as tubes and circles or in more elaborate designs such as sea turtles or alligators. Floating thermometers are a great option because they travel around and read many different parts of the hot tub.