A pool thermometer will not only allow you to monitor your pool's water temperature to ensure it is safe for swimming, it will tell you whether the pool's heating system is in proper working order.
Pool thermometers are relatively inexpensive and come in a variety of styles:
Thermometers come in many shapes, sizes and colors and it is important that you find one that has numbers that can be easily read. Many give the temperature in both Celsius and Fahrenheit, although many differ in the range of temperature it can read.
And remember, it is always wise to check the temperature of the water in different locations of the pool to ensure an accurate reading.
To get a glimpse of the wide assortment of pool thermometers available online, check out some of the shopping links above.