Water temperature varies in general from –2° Celsius to 30° Celsius, or from 28° Fahrenheit to 86° Fahrenheit. But it can rise as high as 40° Celsius/104° Fahrenheit at the equator. Seasonal temperature in the same area may vary by 10° Celsius/18° Fahrenheit.
Temperature is also likely to vary with depth and will often become cooler as you go deeper. A marked change of temperature with depth is called a thermocline. A thermocline separates two layers of water of different temperature 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.
You, as a diver, need to be prepared for the varying temperatures you will encounter on the dive by choosing appropriate exposure protection.