Water temperature varies greatly around the world from –2° Celsius to 30° Celsius, or from 28° Fahrenheit to 86° Fahrenheit. But it can climb as high as 40° Celsius/104° Fahrenheit at the equator. Seasonal temperature in the same area may vary astonishingly. For example, in areas of the Great Lakes (Canada and U.S.), surface temperatures between summer and winter drop more than 20° Celsius/almost 40° Fahrenheit.
Temperature is also likely to vary with depth and will often become cooler as you go deeper. A marked change of water temperature is called a thermocline. A thermocline separates two layers of water of different temperatures (and densities) and occasionally you will get visual distortion where the layers mix. Thermoclines can be at different levels due to many factors including the season and weather conditions.
As a diver, you need to be prepared for the varying temperatures you will encounter on your dives by choosing appropriate exposure protection. A simple guideline is to “dress for depth” and not for surface temperatures.